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<section id="disk-images">
<span id="id1"></span><h1>Disk Images<a class="headerlink" href="#disk-images" title="Link to this heading"></a></h1>
<p>QEMU supports many disk image formats, including growable disk images
(their size increase as non empty sectors are written), compressed and
encrypted disk images.</p>
<section id="quick-start-for-disk-image-creation">
<span id="disk-005fimages-005fquickstart"></span><h2>Quick start for disk image creation<a class="headerlink" href="#quick-start-for-disk-image-creation" title="Link to this heading"></a></h2>
<p>You can create a disk image with the command:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">qemu</span><span class="o">-</span><span class="n">img</span> <span class="n">create</span> <span class="n">myimage</span><span class="o">.</span><span class="n">img</span> <span class="n">mysize</span>
</pre></div>
</div>
<p>where myimage.img is the disk image filename and mysize is its size in
kilobytes. You can add an <code class="docutils literal notranslate"><span class="pre">M</span></code> suffix to give the size in megabytes and
a <code class="docutils literal notranslate"><span class="pre">G</span></code> suffix for gigabytes.</p>
<p>See the <code class="docutils literal notranslate"><span class="pre">qemu-img</span></code> invocation documentation for more information.</p>
</section>
<section id="snapshot-mode">
<span id="disk-005fimages-005fsnapshot-005fmode"></span><h2>Snapshot mode<a class="headerlink" href="#snapshot-mode" title="Link to this heading"></a></h2>
<p>If you use the option <code class="docutils literal notranslate"><span class="pre">-snapshot</span></code>, all disk images are considered as
read only. When sectors in written, they are written in a temporary file
created in <code class="docutils literal notranslate"><span class="pre">/tmp</span></code>. You can however force the write back to the raw
disk images by using the <code class="docutils literal notranslate"><span class="pre">commit</span></code> monitor command (or C-a s in the
serial console).</p>
</section>
<section id="vm-snapshots">
<span id="vm-005fsnapshots"></span><h2>VM snapshots<a class="headerlink" href="#vm-snapshots" title="Link to this heading"></a></h2>
<p>VM snapshots are snapshots of the complete virtual machine including CPU
state, RAM, device state and the content of all the writable disks. In
order to use VM snapshots, you must have at least one non removable and
writable block device using the <code class="docutils literal notranslate"><span class="pre">qcow2</span></code> disk image format. Normally
this device is the first virtual hard drive.</p>
<p>Use the monitor command <code class="docutils literal notranslate"><span class="pre">savevm</span></code> to create a new VM snapshot or
replace an existing one. A human readable name can be assigned to each
snapshot in addition to its numerical ID.</p>
<p>Use <code class="docutils literal notranslate"><span class="pre">loadvm</span></code> to restore a VM snapshot and <code class="docutils literal notranslate"><span class="pre">delvm</span></code> to remove a VM
snapshot. <code class="docutils literal notranslate"><span class="pre">info</span> <span class="pre">snapshots</span></code> lists the available snapshots with their
associated information:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">qemu</span><span class="p">)</span> <span class="n">info</span> <span class="n">snapshots</span>
<span class="n">Snapshot</span> <span class="n">devices</span><span class="p">:</span> <span class="n">hda</span>
<span class="n">Snapshot</span> <span class="nb">list</span> <span class="p">(</span><span class="kn">from</span> <span class="nn">hda</span><span class="p">):</span>
<span class="n">ID</span> <span class="n">TAG</span> <span class="n">VM</span> <span class="n">SIZE</span> <span class="n">DATE</span> <span class="n">VM</span> <span class="n">CLOCK</span>
<span class="mi">1</span> <span class="n">start</span> <span class="mi">41</span><span class="n">M</span> <span class="mi">2006</span><span class="o">-</span><span class="mi">08</span><span class="o">-</span><span class="mi">06</span> <span class="mi">12</span><span class="p">:</span><span class="mi">38</span><span class="p">:</span><span class="mi">02</span> <span class="mi">00</span><span class="p">:</span><span class="mi">00</span><span class="p">:</span><span class="mf">14.954</span>
<span class="mi">2</span> <span class="mi">40</span><span class="n">M</span> <span class="mi">2006</span><span class="o">-</span><span class="mi">08</span><span class="o">-</span><span class="mi">06</span> <span class="mi">12</span><span class="p">:</span><span class="mi">43</span><span class="p">:</span><span class="mi">29</span> <span class="mi">00</span><span class="p">:</span><span class="mi">00</span><span class="p">:</span><span class="mf">18.633</span>
<span class="mi">3</span> <span class="n">msys</span> <span class="mi">40</span><span class="n">M</span> <span class="mi">2006</span><span class="o">-</span><span class="mi">08</span><span class="o">-</span><span class="mi">06</span> <span class="mi">12</span><span class="p">:</span><span class="mi">44</span><span class="p">:</span><span class="mi">04</span> <span class="mi">00</span><span class="p">:</span><span class="mi">00</span><span class="p">:</span><span class="mf">23.514</span>
</pre></div>
</div>
<p>A VM snapshot is made of a VM state info (its size is shown in
<code class="docutils literal notranslate"><span class="pre">info</span> <span class="pre">snapshots</span></code>) and a snapshot of every writable disk image. The VM
state info is stored in the first <code class="docutils literal notranslate"><span class="pre">qcow2</span></code> non removable and writable
block device. The disk image snapshots are stored in every disk image.
The size of a snapshot in a disk image is difficult to evaluate and is
not shown by <code class="docutils literal notranslate"><span class="pre">info</span> <span class="pre">snapshots</span></code> because the associated disk sectors are
shared among all the snapshots to save disk space (otherwise each
snapshot would need a full copy of all the disk images).</p>
<p>When using the (unrelated) <code class="docutils literal notranslate"><span class="pre">-snapshot</span></code> option
(<a class="reference internal" href="#disk-005fimages-005fsnapshot-005fmode"><span class="std std-ref">Snapshot mode</span></a>),
you can always make VM snapshots, but they are deleted as soon as you
exit QEMU.</p>
<p>VM snapshots currently have the following known limitations:</p>
<ul class="simple">
<li><p>They cannot cope with removable devices if they are removed or
inserted after a snapshot is done.</p></li>
<li><p>A few device drivers still have incomplete snapshot support so their
state is not saved or restored properly (in particular USB).</p></li>
</ul>
</section>
<section id="disk-image-file-formats">
<h2>Disk image file formats<a class="headerlink" href="#disk-image-file-formats" title="Link to this heading"></a></h2>
<p>QEMU supports many image file formats that can be used with VMs as well as with
any of the tools (like <code class="docutils literal notranslate"><span class="pre">qemu-img</span></code>). This includes the preferred formats
raw and qcow2 as well as formats that are supported for compatibility with
older QEMU versions or other hypervisors.</p>
<p>Depending on the image format, different options can be passed to
<code class="docutils literal notranslate"><span class="pre">qemu-img</span> <span class="pre">create</span></code> and <code class="docutils literal notranslate"><span class="pre">qemu-img</span> <span class="pre">convert</span></code> using the <code class="docutils literal notranslate"><span class="pre">-o</span></code> option.
This section describes each format and the options that are supported for it.</p>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-image-formats-arg-raw">
<span class="sig-name descname"><span class="pre">raw</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-image-formats-arg-raw" title="Link to this definition"></a></dt>
<dd><p>Raw disk image format. This format has the advantage of
being simple and easily exportable to all other emulators. If your
file system supports <em>holes</em> (for example in ext2 or ext3 on
Linux or NTFS on Windows), then only the written sectors will reserve
space. Use <code class="docutils literal notranslate"><span class="pre">qemu-img</span> <span class="pre">info</span></code> to know the real size used by the
image or <code class="docutils literal notranslate"><span class="pre">ls</span> <span class="pre">-ls</span></code> on Unix/Linux.</p>
<p>Supported options:</p>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-raw-arg-preallocation">
<span class="sig-name descname"><span class="pre">preallocation</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-raw-arg-preallocation" title="Link to this definition"></a></dt>
<dd><p>Preallocation mode (allowed values: <code class="docutils literal notranslate"><span class="pre">off</span></code>, <code class="docutils literal notranslate"><span class="pre">falloc</span></code>,
<code class="docutils literal notranslate"><span class="pre">full</span></code>). <code class="docutils literal notranslate"><span class="pre">falloc</span></code> mode preallocates space for image by
calling <code class="docutils literal notranslate"><span class="pre">posix_fallocate()</span></code>. <code class="docutils literal notranslate"><span class="pre">full</span></code> mode preallocates space
for image by writing data to underlying storage. This data may or
may not be zero, depending on the storage location.</p>
</dd></dl>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-image-formats-arg-qcow2">
<span class="sig-name descname"><span class="pre">qcow2</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-image-formats-arg-qcow2" title="Link to this definition"></a></dt>
<dd><p>QEMU image format, the most versatile format. Use it to have smaller
images (useful if your filesystem does not supports holes, for example
on Windows), zlib based compression and support of multiple VM
snapshots.</p>
<p>Supported options:</p>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-qcow2-arg-compat">
<span class="sig-name descname"><span class="pre">compat</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-qcow2-arg-compat" title="Link to this definition"></a></dt>
<dd><p>Determines the qcow2 version to use. <code class="docutils literal notranslate"><span class="pre">compat=0.10</span></code> uses the
traditional image format that can be read by any QEMU since 0.10.
<code class="docutils literal notranslate"><span class="pre">compat=1.1</span></code> enables image format extensions that only QEMU 1.1 and
newer understand (this is the default). Amongst others, this includes
zero clusters, which allow efficient copy-on-read for sparse images.</p>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-qcow2-arg-backing_file">
<span class="sig-name descname"><span class="pre">backing_file</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-qcow2-arg-backing_file" title="Link to this definition"></a></dt>
<dd><p>File name of a base image (see <code class="docutils literal notranslate"><span class="pre">create</span></code> subcommand)</p>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-qcow2-arg-backing_fmt">
<span class="sig-name descname"><span class="pre">backing_fmt</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-qcow2-arg-backing_fmt" title="Link to this definition"></a></dt>
<dd><p>Image format of the base image</p>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-qcow2-arg-encryption">
<span class="sig-name descname"><span class="pre">encryption</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-qcow2-arg-encryption" title="Link to this definition"></a></dt>
<dd><p>This option is deprecated and equivalent to <code class="docutils literal notranslate"><span class="pre">encrypt.format=aes</span></code></p>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-qcow2-arg-encrypt.format">
<span class="sig-name descname"><span class="pre">encrypt.format</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-qcow2-arg-encrypt.format" title="Link to this definition"></a></dt>
<dd><p>If this is set to <code class="docutils literal notranslate"><span class="pre">luks</span></code>, it requests that the qcow2 payload (not
qcow2 header) be encrypted using the LUKS format. The passphrase to
use to unlock the LUKS key slot is given by the <code class="docutils literal notranslate"><span class="pre">encrypt.key-secret</span></code>
parameter. LUKS encryption parameters can be tuned with the other
<code class="docutils literal notranslate"><span class="pre">encrypt.*</span></code> parameters.</p>
<p>If this is set to <code class="docutils literal notranslate"><span class="pre">aes</span></code>, the image is encrypted with 128-bit AES-CBC.
The encryption key is given by the <code class="docutils literal notranslate"><span class="pre">encrypt.key-secret</span></code> parameter.
This encryption format is considered to be flawed by modern cryptography
standards, suffering from a number of design problems:</p>
<ul class="simple">
<li><p>The AES-CBC cipher is used with predictable initialization vectors based
on the sector number. This makes it vulnerable to chosen plaintext attacks
which can reveal the existence of encrypted data.</p></li>
<li><p>The user passphrase is directly used as the encryption key. A poorly
chosen or short passphrase will compromise the security of the encryption.</p></li>
<li><p>In the event of the passphrase being compromised there is no way to
change the passphrase to protect data in any qcow images. The files must
be cloned, using a different encryption passphrase in the new file. The
original file must then be securely erased using a program like shred,
though even this is ineffective with many modern storage technologies.</p></li>
</ul>
<p>The use of this is no longer supported in system emulators. Support only
remains in the command line utilities, for the purposes of data liberation
and interoperability with old versions of QEMU. The <code class="docutils literal notranslate"><span class="pre">luks</span></code> format
should be used instead.</p>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-qcow2-arg-encrypt.key-secret">
<span class="sig-name descname"><span class="pre">encrypt.key-secret</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-qcow2-arg-encrypt.key-secret" title="Link to this definition"></a></dt>
<dd><p>Provides the ID of a <code class="docutils literal notranslate"><span class="pre">secret</span></code> object that contains the passphrase
(<code class="docutils literal notranslate"><span class="pre">encrypt.format=luks</span></code>) or encryption key (<code class="docutils literal notranslate"><span class="pre">encrypt.format=aes</span></code>).</p>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-qcow2-arg-encrypt.cipher-alg">
<span class="sig-name descname"><span class="pre">encrypt.cipher-alg</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-qcow2-arg-encrypt.cipher-alg" title="Link to this definition"></a></dt>
<dd><p>Name of the cipher algorithm and key length. Currently defaults
to <code class="docutils literal notranslate"><span class="pre">aes-256</span></code>. Only used when <code class="docutils literal notranslate"><span class="pre">encrypt.format=luks</span></code>.</p>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-qcow2-arg-encrypt.cipher-mode">
<span class="sig-name descname"><span class="pre">encrypt.cipher-mode</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-qcow2-arg-encrypt.cipher-mode" title="Link to this definition"></a></dt>
<dd><p>Name of the encryption mode to use. Currently defaults to <code class="docutils literal notranslate"><span class="pre">xts</span></code>.
Only used when <code class="docutils literal notranslate"><span class="pre">encrypt.format=luks</span></code>.</p>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-qcow2-arg-encrypt.ivgen-alg">
<span class="sig-name descname"><span class="pre">encrypt.ivgen-alg</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-qcow2-arg-encrypt.ivgen-alg" title="Link to this definition"></a></dt>
<dd><p>Name of the initialization vector generator algorithm. Currently defaults
to <code class="docutils literal notranslate"><span class="pre">plain64</span></code>. Only used when <code class="docutils literal notranslate"><span class="pre">encrypt.format=luks</span></code>.</p>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-qcow2-arg-encrypt.ivgen-hash-alg">
<span class="sig-name descname"><span class="pre">encrypt.ivgen-hash-alg</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-qcow2-arg-encrypt.ivgen-hash-alg" title="Link to this definition"></a></dt>
<dd><p>Name of the hash algorithm to use with the initialization vector generator
(if required). Defaults to <code class="docutils literal notranslate"><span class="pre">sha256</span></code>. Only used when <code class="docutils literal notranslate"><span class="pre">encrypt.format=luks</span></code>.</p>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-qcow2-arg-encrypt.hash-alg">
<span class="sig-name descname"><span class="pre">encrypt.hash-alg</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-qcow2-arg-encrypt.hash-alg" title="Link to this definition"></a></dt>
<dd><p>Name of the hash algorithm to use for PBKDF algorithm
Defaults to <code class="docutils literal notranslate"><span class="pre">sha256</span></code>. Only used when <code class="docutils literal notranslate"><span class="pre">encrypt.format=luks</span></code>.</p>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-qcow2-arg-encrypt.iter-time">
<span class="sig-name descname"><span class="pre">encrypt.iter-time</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-qcow2-arg-encrypt.iter-time" title="Link to this definition"></a></dt>
<dd><p>Amount of time, in milliseconds, to use for PBKDF algorithm per key slot.
Defaults to <code class="docutils literal notranslate"><span class="pre">2000</span></code>. Only used when <code class="docutils literal notranslate"><span class="pre">encrypt.format=luks</span></code>.</p>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-qcow2-arg-cluster_size">
<span class="sig-name descname"><span class="pre">cluster_size</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-qcow2-arg-cluster_size" title="Link to this definition"></a></dt>
<dd><p>Changes the qcow2 cluster size (must be between 512 and 2M). Smaller cluster
sizes can improve the image file size whereas larger cluster sizes generally
provide better performance.</p>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-qcow2-arg-preallocation">
<span class="sig-name descname"><span class="pre">preallocation</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-qcow2-arg-preallocation" title="Link to this definition"></a></dt>
<dd><p>Preallocation mode (allowed values: <code class="docutils literal notranslate"><span class="pre">off</span></code>, <code class="docutils literal notranslate"><span class="pre">metadata</span></code>, <code class="docutils literal notranslate"><span class="pre">falloc</span></code>,
<code class="docutils literal notranslate"><span class="pre">full</span></code>). An image with preallocated metadata is initially larger but can
improve performance when the image needs to grow. <code class="docutils literal notranslate"><span class="pre">falloc</span></code> and <code class="docutils literal notranslate"><span class="pre">full</span></code>
preallocations are like the same options of <code class="docutils literal notranslate"><span class="pre">raw</span></code> format, but sets up
metadata also.</p>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-qcow2-arg-lazy_refcounts">
<span class="sig-name descname"><span class="pre">lazy_refcounts</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-qcow2-arg-lazy_refcounts" title="Link to this definition"></a></dt>
<dd><p>If this option is set to <code class="docutils literal notranslate"><span class="pre">on</span></code>, reference count updates are postponed with
the goal of avoiding metadata I/O and improving performance. This is
particularly interesting with <code class="xref std std-option docutils literal notranslate"><span class="pre">cache=writethrough</span></code> which doesn’t batch
metadata updates. The tradeoff is that after a host crash, the reference count
tables must be rebuilt, i.e. on the next open an (automatic) <code class="docutils literal notranslate"><span class="pre">qemu-img</span>
<span class="pre">check</span> <span class="pre">-r</span> <span class="pre">all</span></code> is required, which may take some time.</p>
<p>This option can only be enabled if <code class="docutils literal notranslate"><span class="pre">compat=1.1</span></code> is specified.</p>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-qcow2-arg-nocow">
<span class="sig-name descname"><span class="pre">nocow</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-qcow2-arg-nocow" title="Link to this definition"></a></dt>
<dd><p>If this option is set to <code class="docutils literal notranslate"><span class="pre">on</span></code>, it will turn off COW of the file. It’s only
valid on btrfs, no effect on other file systems.</p>
<p>Btrfs has low performance when hosting a VM image file, even more
when the guest on the VM also using btrfs as file system. Turning off
COW is a way to mitigate this bad performance. Generally there are two
ways to turn off COW on btrfs:</p>
<ul class="simple">
<li><p>Disable it by mounting with nodatacow, then all newly created files
will be NOCOW.</p></li>
<li><p>For an empty file, add the NOCOW file attribute. That’s what this
option does.</p></li>
</ul>
<p>Note: this option is only valid to new or empty files. If there is
an existing file which is COW and has data blocks already, it couldn’t
be changed to NOCOW by setting <code class="docutils literal notranslate"><span class="pre">nocow=on</span></code>. One can issue <code class="docutils literal notranslate"><span class="pre">lsattr</span>
<span class="pre">filename</span></code> to check if the NOCOW flag is set or not (Capital ‘C’ is
NOCOW flag).</p>
</dd></dl>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-image-formats-arg-qed">
<span class="sig-name descname"><span class="pre">qed</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-image-formats-arg-qed" title="Link to this definition"></a></dt>
<dd><p>Old QEMU image format with support for backing files and compact image files
(when your filesystem or transport medium does not support holes).</p>
<p>When converting QED images to qcow2, you might want to consider using the
<code class="docutils literal notranslate"><span class="pre">lazy_refcounts=on</span></code> option to get a more QED-like behaviour.</p>
<p>Supported options:</p>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-qed-arg-backing_file">
<span class="sig-name descname"><span class="pre">backing_file</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-qed-arg-backing_file" title="Link to this definition"></a></dt>
<dd><p>File name of a base image (see <code class="docutils literal notranslate"><span class="pre">create</span></code> subcommand).</p>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-qed-arg-backing_fmt">
<span class="sig-name descname"><span class="pre">backing_fmt</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-qed-arg-backing_fmt" title="Link to this definition"></a></dt>
<dd><p>Image file format of backing file (optional). Useful if the format cannot be
autodetected because it has no header, like some vhd/vpc files.</p>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-qed-arg-cluster_size">
<span class="sig-name descname"><span class="pre">cluster_size</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-qed-arg-cluster_size" title="Link to this definition"></a></dt>
<dd><p>Changes the cluster size (must be power-of-2 between 4K and 64K). Smaller
cluster sizes can improve the image file size whereas larger cluster sizes
generally provide better performance.</p>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-qed-arg-table_size">
<span class="sig-name descname"><span class="pre">table_size</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-qed-arg-table_size" title="Link to this definition"></a></dt>
<dd><p>Changes the number of clusters per L1/L2 table (must be
power-of-2 between 1 and 16). There is normally no need to
change this value but this option can between used for
performance benchmarking.</p>
</dd></dl>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-image-formats-arg-qcow">
<span class="sig-name descname"><span class="pre">qcow</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-image-formats-arg-qcow" title="Link to this definition"></a></dt>
<dd><p>Old QEMU image format with support for backing files, compact image files,
encryption and compression.</p>
<p>Supported options:</p>
<blockquote>
<div><dl class="std option">
<dt class="sig sig-object std" id="cmdoption-qcow-arg-backing_file">
<span class="sig-name descname"><span class="pre">backing_file</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-qcow-arg-backing_file" title="Link to this definition"></a></dt>
<dd><p>File name of a base image (see <code class="docutils literal notranslate"><span class="pre">create</span></code> subcommand)</p>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-qcow-arg-encryption">
<span class="sig-name descname"><span class="pre">encryption</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-qcow-arg-encryption" title="Link to this definition"></a></dt>
<dd><p>This option is deprecated and equivalent to <code class="docutils literal notranslate"><span class="pre">encrypt.format=aes</span></code></p>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-qcow-arg-encrypt.format">
<span class="sig-name descname"><span class="pre">encrypt.format</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-qcow-arg-encrypt.format" title="Link to this definition"></a></dt>
<dd><p>If this is set to <code class="docutils literal notranslate"><span class="pre">aes</span></code>, the image is encrypted with 128-bit AES-CBC.
The encryption key is given by the <code class="docutils literal notranslate"><span class="pre">encrypt.key-secret</span></code> parameter.
This encryption format is considered to be flawed by modern cryptography
standards, suffering from a number of design problems enumerated previously
against the <code class="docutils literal notranslate"><span class="pre">qcow2</span></code> image format.</p>
<p>The use of this is no longer supported in system emulators. Support only
remains in the command line utilities, for the purposes of data liberation
and interoperability with old versions of QEMU.</p>
<p>Users requiring native encryption should use the <code class="docutils literal notranslate"><span class="pre">qcow2</span></code> format
instead with <code class="docutils literal notranslate"><span class="pre">encrypt.format=luks</span></code>.</p>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-qcow-arg-encrypt.key-secret">
<span class="sig-name descname"><span class="pre">encrypt.key-secret</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-qcow-arg-encrypt.key-secret" title="Link to this definition"></a></dt>
<dd><p>Provides the ID of a <code class="docutils literal notranslate"><span class="pre">secret</span></code> object that contains the encryption
key (<code class="docutils literal notranslate"><span class="pre">encrypt.format=aes</span></code>).</p>
</dd></dl>
</div></blockquote>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-image-formats-arg-luks">
<span class="sig-name descname"><span class="pre">luks</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-image-formats-arg-luks" title="Link to this definition"></a></dt>
<dd><p>LUKS v1 encryption format, compatible with Linux dm-crypt/cryptsetup</p>
<p>Supported options:</p>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-luks-arg-key-secret">
<span class="sig-name descname"><span class="pre">key-secret</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-luks-arg-key-secret" title="Link to this definition"></a></dt>
<dd><p>Provides the ID of a <code class="docutils literal notranslate"><span class="pre">secret</span></code> object that contains the passphrase.</p>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-luks-arg-cipher-alg">
<span class="sig-name descname"><span class="pre">cipher-alg</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-luks-arg-cipher-alg" title="Link to this definition"></a></dt>
<dd><p>Name of the cipher algorithm and key length. Currently defaults
to <code class="docutils literal notranslate"><span class="pre">aes-256</span></code>.</p>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-luks-arg-cipher-mode">
<span class="sig-name descname"><span class="pre">cipher-mode</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-luks-arg-cipher-mode" title="Link to this definition"></a></dt>
<dd><p>Name of the encryption mode to use. Currently defaults to <code class="docutils literal notranslate"><span class="pre">xts</span></code>.</p>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-luks-arg-ivgen-alg">
<span class="sig-name descname"><span class="pre">ivgen-alg</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-luks-arg-ivgen-alg" title="Link to this definition"></a></dt>
<dd><p>Name of the initialization vector generator algorithm. Currently defaults
to <code class="docutils literal notranslate"><span class="pre">plain64</span></code>.</p>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-luks-arg-ivgen-hash-alg">
<span class="sig-name descname"><span class="pre">ivgen-hash-alg</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-luks-arg-ivgen-hash-alg" title="Link to this definition"></a></dt>
<dd><p>Name of the hash algorithm to use with the initialization vector generator
(if required). Defaults to <code class="docutils literal notranslate"><span class="pre">sha256</span></code>.</p>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-luks-arg-hash-alg">
<span class="sig-name descname"><span class="pre">hash-alg</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-luks-arg-hash-alg" title="Link to this definition"></a></dt>
<dd><p>Name of the hash algorithm to use for PBKDF algorithm
Defaults to <code class="docutils literal notranslate"><span class="pre">sha256</span></code>.</p>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-luks-arg-iter-time">
<span class="sig-name descname"><span class="pre">iter-time</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-luks-arg-iter-time" title="Link to this definition"></a></dt>
<dd><p>Amount of time, in milliseconds, to use for PBKDF algorithm per key slot.
Defaults to <code class="docutils literal notranslate"><span class="pre">2000</span></code>.</p>
</dd></dl>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-image-formats-arg-vdi">
<span class="sig-name descname"><span class="pre">vdi</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-image-formats-arg-vdi" title="Link to this definition"></a></dt>
<dd><p>VirtualBox 1.1 compatible image format.</p>
<p>Supported options:</p>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-vdi-arg-static">
<span class="sig-name descname"><span class="pre">static</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-vdi-arg-static" title="Link to this definition"></a></dt>
<dd><p>If this option is set to <code class="docutils literal notranslate"><span class="pre">on</span></code>, the image is created with metadata
preallocation.</p>
</dd></dl>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-image-formats-arg-vmdk">
<span class="sig-name descname"><span class="pre">vmdk</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-image-formats-arg-vmdk" title="Link to this definition"></a></dt>
<dd><p>VMware 3 and 4 compatible image format.</p>
<p>Supported options:</p>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-image-formats-arg-backing_file">
<span class="sig-name descname"><span class="pre">backing_file</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-image-formats-arg-backing_file" title="Link to this definition"></a></dt>
<dd><p>File name of a base image (see <code class="docutils literal notranslate"><span class="pre">create</span></code> subcommand).</p>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-image-formats-arg-compat6">
<span class="sig-name descname"><span class="pre">compat6</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-image-formats-arg-compat6" title="Link to this definition"></a></dt>
<dd><p>Create a VMDK version 6 image (instead of version 4)</p>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-image-formats-arg-hwversion">
<span class="sig-name descname"><span class="pre">hwversion</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-image-formats-arg-hwversion" title="Link to this definition"></a></dt>
<dd><p>Specify vmdk virtual hardware version. Compat6 flag cannot be enabled
if hwversion is specified.</p>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-image-formats-arg-subformat">
<span class="sig-name descname"><span class="pre">subformat</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-image-formats-arg-subformat" title="Link to this definition"></a></dt>
<dd><p>Specifies which VMDK subformat to use. Valid options are
<code class="docutils literal notranslate"><span class="pre">monolithicSparse</span></code> (default),
<code class="docutils literal notranslate"><span class="pre">monolithicFlat</span></code>,
<code class="docutils literal notranslate"><span class="pre">twoGbMaxExtentSparse</span></code>,
<code class="docutils literal notranslate"><span class="pre">twoGbMaxExtentFlat</span></code> and
<code class="docutils literal notranslate"><span class="pre">streamOptimized</span></code>.</p>
</dd></dl>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-image-formats-arg-vpc">
<span class="sig-name descname"><span class="pre">vpc</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-image-formats-arg-vpc" title="Link to this definition"></a></dt>
<dd><p>VirtualPC compatible image format (VHD).</p>
<p>Supported options:</p>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-vpc-arg-subformat">
<span class="sig-name descname"><span class="pre">subformat</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-vpc-arg-subformat" title="Link to this definition"></a></dt>
<dd><p>Specifies which VHD subformat to use. Valid options are
<code class="docutils literal notranslate"><span class="pre">dynamic</span></code> (default) and <code class="docutils literal notranslate"><span class="pre">fixed</span></code>.</p>
</dd></dl>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-image-formats-arg-VHDX">
<span class="sig-name descname"><span class="pre">VHDX</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-image-formats-arg-VHDX" title="Link to this definition"></a></dt>
<dd><p>Hyper-V compatible image format (VHDX).</p>
<p>Supported options:</p>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-VHDX-arg-subformat">
<span class="sig-name descname"><span class="pre">subformat</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-VHDX-arg-subformat" title="Link to this definition"></a></dt>
<dd><p>Specifies which VHDX subformat to use. Valid options are
<code class="docutils literal notranslate"><span class="pre">dynamic</span></code> (default) and <code class="docutils literal notranslate"><span class="pre">fixed</span></code>.</p>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-VHDX-arg-block_state_zero">
<span class="sig-name descname"><span class="pre">block_state_zero</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-VHDX-arg-block_state_zero" title="Link to this definition"></a></dt>
<dd><p>Force use of payload blocks of type ‘ZERO’. Can be set to <code class="docutils literal notranslate"><span class="pre">on</span></code> (default)
or <code class="docutils literal notranslate"><span class="pre">off</span></code>. When set to <code class="docutils literal notranslate"><span class="pre">off</span></code>, new blocks will be created as
<code class="docutils literal notranslate"><span class="pre">PAYLOAD_BLOCK_NOT_PRESENT</span></code>, which means parsers are free to return
arbitrary data for those blocks. Do not set to <code class="docutils literal notranslate"><span class="pre">off</span></code> when using
<code class="docutils literal notranslate"><span class="pre">qemu-img</span> <span class="pre">convert</span></code> with <code class="docutils literal notranslate"><span class="pre">subformat=dynamic</span></code>.</p>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-VHDX-arg-block_size">
<span class="sig-name descname"><span class="pre">block_size</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-VHDX-arg-block_size" title="Link to this definition"></a></dt>
<dd><p>Block size; min 1 MB, max 256 MB. 0 means auto-calculate based on
image size.</p>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-VHDX-arg-log_size">
<span class="sig-name descname"><span class="pre">log_size</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-VHDX-arg-log_size" title="Link to this definition"></a></dt>
<dd><p>Log size; min 1 MB.</p>
</dd></dl>
</dd></dl>
</dd></dl>
</section>
<section id="read-only-formats">
<h2>Read-only formats<a class="headerlink" href="#read-only-formats" title="Link to this heading"></a></h2>
<p>More disk image file formats are supported in a read-only mode.</p>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-image-formats-arg-bochs">
<span class="sig-name descname"><span class="pre">bochs</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-image-formats-arg-bochs" title="Link to this definition"></a></dt>
<dd><p>Bochs images of <code class="docutils literal notranslate"><span class="pre">growing</span></code> type.</p>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-image-formats-arg-cloop">
<span class="sig-name descname"><span class="pre">cloop</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-image-formats-arg-cloop" title="Link to this definition"></a></dt>
<dd><p>Linux Compressed Loop image, useful only to reuse directly compressed
CD-ROM images present for example in the Knoppix CD-ROMs.</p>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-image-formats-arg-dmg">
<span class="sig-name descname"><span class="pre">dmg</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-image-formats-arg-dmg" title="Link to this definition"></a></dt>
<dd><p>Apple disk image.</p>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-image-formats-arg-parallels">
<span class="sig-name descname"><span class="pre">parallels</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-image-formats-arg-parallels" title="Link to this definition"></a></dt>
<dd><p>Parallels disk image format.</p>
</dd></dl>
</section>
<section id="using-host-drives">
<h2>Using host drives<a class="headerlink" href="#using-host-drives" title="Link to this heading"></a></h2>
<p>In addition to disk image files, QEMU can directly access host
devices. We describe here the usage for QEMU version &gt;= 0.8.3.</p>
<section id="linux">
<h3>Linux<a class="headerlink" href="#linux" title="Link to this heading"></a></h3>
<p>On Linux, you can directly use the host device filename instead of a
disk image filename provided you have enough privileges to access
it. For example, use <code class="docutils literal notranslate"><span class="pre">/dev/cdrom</span></code> to access to the CDROM.</p>
<dl class="simple">
<dt>CD</dt><dd><p>You can specify a CDROM device even if no CDROM is loaded. QEMU has
specific code to detect CDROM insertion or removal. CDROM ejection by
the guest OS is supported. Currently only data CDs are supported.</p>
</dd>
<dt>Floppy</dt><dd><p>You can specify a floppy device even if no floppy is loaded. Floppy
removal is currently not detected accurately (if you change floppy
without doing floppy access while the floppy is not loaded, the guest
OS will think that the same floppy is loaded).
Use of the host’s floppy device is deprecated, and support for it will
be removed in a future release.</p>
</dd>
<dt>Hard disks</dt><dd><p>Hard disks can be used. Normally you must specify the whole disk
(<code class="docutils literal notranslate"><span class="pre">/dev/hdb</span></code> instead of <code class="docutils literal notranslate"><span class="pre">/dev/hdb1</span></code>) so that the guest OS can
see it as a partitioned disk. WARNING: unless you know what you do, it
is better to only make READ-ONLY accesses to the hard disk otherwise
you may corrupt your host data (use the <code class="docutils literal notranslate"><span class="pre">-snapshot</span></code> command
line option or modify the device permissions accordingly).</p>
</dd>
<dt>Zoned block devices</dt><dd><p>Zoned block devices can be passed through to the guest if the emulated storage
controller supports zoned storage. Use <code class="docutils literal notranslate"><span class="pre">--blockdev</span> <span class="pre">host_device,</span>
<span class="pre">node-name=drive0,filename=/dev/nullb0,cache.direct=on</span></code> to pass through
<code class="docutils literal notranslate"><span class="pre">/dev/nullb0</span></code> as <code class="docutils literal notranslate"><span class="pre">drive0</span></code>.</p>
</dd>
</dl>
</section>
<section id="windows">
<h3>Windows<a class="headerlink" href="#windows" title="Link to this heading"></a></h3>
<dl>
<dt>CD</dt><dd><p>The preferred syntax is the drive letter (e.g. <code class="docutils literal notranslate"><span class="pre">d:</span></code>). The
alternate syntax <code class="docutils literal notranslate"><span class="pre">\\.\d:</span></code> is supported. <code class="docutils literal notranslate"><span class="pre">/dev/cdrom</span></code> is
supported as an alias to the first CDROM drive.</p>
<p>Currently there is no specific code to handle removable media, so it
is better to use the <code class="docutils literal notranslate"><span class="pre">change</span></code> or <code class="docutils literal notranslate"><span class="pre">eject</span></code> monitor commands to
change or eject media.</p>
</dd>
<dt>Hard disks</dt><dd><p>Hard disks can be used with the syntax: <code class="docutils literal notranslate"><span class="pre">\\.\PhysicalDriveN</span></code>
where <em>N</em> is the drive number (0 is the first hard disk).</p>
<p>WARNING: unless you know what you do, it is better to only make
READ-ONLY accesses to the hard disk otherwise you may corrupt your
host data (use the <code class="docutils literal notranslate"><span class="pre">-snapshot</span></code> command line so that the
modifications are written in a temporary file).</p>
</dd>
</dl>
</section>
<section id="mac-os-x">
<h3>Mac OS X<a class="headerlink" href="#mac-os-x" title="Link to this heading"></a></h3>
<p><code class="docutils literal notranslate"><span class="pre">/dev/cdrom</span></code> is an alias to the first CDROM.</p>
<p>Currently there is no specific code to handle removable media, so it
is better to use the <code class="docutils literal notranslate"><span class="pre">change</span></code> or <code class="docutils literal notranslate"><span class="pre">eject</span></code> monitor commands to
change or eject media.</p>
</section>
</section>
<section id="virtual-fat-disk-images">
<h2>Virtual FAT disk images<a class="headerlink" href="#virtual-fat-disk-images" title="Link to this heading"></a></h2>
<p>QEMU can automatically create a virtual FAT disk image from a
directory tree. In order to use it, just type:</p>
<pre class="literal-block">qemu-system-x86_64 linux.img -hdb fat:/my_directory</pre>
<p>Then you access access to all the files in the <code class="docutils literal notranslate"><span class="pre">/my_directory</span></code>
directory without having to copy them in a disk image or to export
them via SAMBA or NFS. The default access is <em>read-only</em>.</p>
<p>Floppies can be emulated with the <code class="docutils literal notranslate"><span class="pre">:floppy:</span></code> option:</p>
<pre class="literal-block">qemu-system-x86_64 linux.img -fda fat:floppy:/my_directory</pre>
<p>A read/write support is available for testing (beta stage) with the
<code class="docutils literal notranslate"><span class="pre">:rw:</span></code> option:</p>
<pre class="literal-block">qemu-system-x86_64 linux.img -fda fat:floppy:rw:/my_directory</pre>
<p>What you should <em>never</em> do:</p>
<ul class="simple">
<li><p>use non-ASCII filenames</p></li>
<li><p>use “-snapshot” together with “:rw:”</p></li>
<li><p>expect it to work when loadvm’ing</p></li>
<li><p>write to the FAT directory on the host system while accessing it with the guest system</p></li>
</ul>
</section>
<section id="nbd-access">
<h2>NBD access<a class="headerlink" href="#nbd-access" title="Link to this heading"></a></h2>
<p>QEMU can access directly to block device exported using the Network Block Device
protocol.</p>
<pre class="literal-block">qemu-system-x86_64 linux.img -hdb nbd://my_nbd_server.mydomain.org:1024/</pre>
<p>If the NBD server is located on the same host, you can use an unix socket instead
of an inet socket:</p>
<pre class="literal-block">qemu-system-x86_64 linux.img -hdb nbd+unix://?socket=/tmp/my_socket</pre>
<p>In this case, the block device must be exported using <code class="docutils literal notranslate"><span class="pre">qemu-nbd</span></code>:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">qemu</span><span class="o">-</span><span class="n">nbd</span> <span class="o">--</span><span class="n">socket</span><span class="o">=/</span><span class="n">tmp</span><span class="o">/</span><span class="n">my_socket</span> <span class="n">my_disk</span><span class="o">.</span><span class="n">qcow2</span>
</pre></div>
</div>
<p>The use of <code class="docutils literal notranslate"><span class="pre">qemu-nbd</span></code> allows sharing of a disk between several guests:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">qemu</span><span class="o">-</span><span class="n">nbd</span> <span class="o">--</span><span class="n">socket</span><span class="o">=/</span><span class="n">tmp</span><span class="o">/</span><span class="n">my_socket</span> <span class="o">--</span><span class="n">share</span><span class="o">=</span><span class="mi">2</span> <span class="n">my_disk</span><span class="o">.</span><span class="n">qcow2</span>
</pre></div>
</div>
<p>and then you can use it with two guests:</p>
<pre class="literal-block">qemu-system-x86_64 linux1.img -hdb nbd+unix://?socket=/tmp/my_socket
qemu-system-x86_64 linux2.img -hdb nbd+unix://?socket=/tmp/my_socket</pre>
<p>If the <code class="docutils literal notranslate"><span class="pre">nbd-server</span></code> uses named exports (supported since NBD 2.9.18, or with QEMU’s
own embedded NBD server), you must specify an export name in the URI:</p>
<pre class="literal-block">qemu-system-x86_64 -cdrom nbd://localhost/debian-500-ppc-netinst
qemu-system-x86_64 -cdrom nbd://localhost/openSUSE-11.1-ppc-netinst</pre>
<p>The URI syntax for NBD is supported since QEMU 1.3. An alternative syntax is
also available. Here are some example of the older syntax:</p>
<pre class="literal-block">qemu-system-x86_64 linux.img -hdb nbd:my_nbd_server.mydomain.org:1024
qemu-system-x86_64 linux2.img -hdb nbd:unix:/tmp/my_socket
qemu-system-x86_64 -cdrom nbd:localhost:10809:exportname=debian-500-ppc-netinst</pre>
</section>
<section id="iscsi-luns">
<h2>iSCSI LUNs<a class="headerlink" href="#iscsi-luns" title="Link to this heading"></a></h2>
<p>iSCSI is a popular protocol used to access SCSI devices across a computer
network.</p>
<p>There are two different ways iSCSI devices can be used by QEMU.</p>
<p>The first method is to mount the iSCSI LUN on the host, and make it appear as
any other ordinary SCSI device on the host and then to access this device as a
/dev/sd device from QEMU. How to do this differs between host OSes.</p>
<p>The second method involves using the iSCSI initiator that is built into
QEMU. This provides a mechanism that works the same way regardless of which
host OS you are running QEMU on. This section will describe this second method
of using iSCSI together with QEMU.</p>
<p>In QEMU, iSCSI devices are described using special iSCSI URLs. URL syntax:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">iscsi</span><span class="p">:</span><span class="o">//</span><span class="p">[</span><span class="o">&lt;</span><span class="n">username</span><span class="o">&gt;</span><span class="p">[</span><span class="o">%&lt;</span><span class="n">password</span><span class="o">&gt;</span><span class="p">]</span><span class="o">@</span><span class="p">]</span><span class="o">&lt;</span><span class="n">host</span><span class="o">&gt;</span><span class="p">[:</span><span class="o">&lt;</span><span class="n">port</span><span class="o">&gt;</span><span class="p">]</span><span class="o">/&lt;</span><span class="n">target</span><span class="o">-</span><span class="n">iqn</span><span class="o">-</span><span class="n">name</span><span class="o">&gt;/&lt;</span><span class="n">lun</span><span class="o">&gt;</span>
</pre></div>
</div>
<p>Username and password are optional and only used if your target is set up
using CHAP authentication for access control.
Alternatively the username and password can also be set via environment
variables to have these not show up in the process list:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">export</span> <span class="n">LIBISCSI_CHAP_USERNAME</span><span class="o">=&lt;</span><span class="n">username</span><span class="o">&gt;</span>
<span class="n">export</span> <span class="n">LIBISCSI_CHAP_PASSWORD</span><span class="o">=&lt;</span><span class="n">password</span><span class="o">&gt;</span>
<span class="n">iscsi</span><span class="p">:</span><span class="o">//&lt;</span><span class="n">host</span><span class="o">&gt;/&lt;</span><span class="n">target</span><span class="o">-</span><span class="n">iqn</span><span class="o">-</span><span class="n">name</span><span class="o">&gt;/&lt;</span><span class="n">lun</span><span class="o">&gt;</span>
</pre></div>
</div>
<p>Various session related parameters can be set via special options, either
in a configuration file provided via ‘-readconfig’ or directly on the
command line.</p>
<p>If the initiator-name is not specified qemu will use a default name
of ‘iqn.2008-11.org.linux-kvm[:&lt;uuid&gt;’] where &lt;uuid&gt; is the UUID of the
virtual machine. If the UUID is not specified qemu will use
‘iqn.2008-11.org.linux-kvm[:&lt;name&gt;’] where &lt;name&gt; is the name of the
virtual machine.</p>
<p>Setting a specific initiator name to use when logging in to the target:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">-</span><span class="n">iscsi</span> <span class="n">initiator</span><span class="o">-</span><span class="n">name</span><span class="o">=</span><span class="n">iqn</span><span class="o">.</span><span class="n">qemu</span><span class="o">.</span><span class="n">test</span><span class="p">:</span><span class="n">my</span><span class="o">-</span><span class="n">initiator</span>
</pre></div>
</div>
<p>Controlling which type of header digest to negotiate with the target:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">-</span><span class="n">iscsi</span> <span class="n">header</span><span class="o">-</span><span class="n">digest</span><span class="o">=</span><span class="n">CRC32C</span><span class="o">|</span><span class="n">CRC32C</span><span class="o">-</span><span class="n">NONE</span><span class="o">|</span><span class="n">NONE</span><span class="o">-</span><span class="n">CRC32C</span><span class="o">|</span><span class="n">NONE</span>
</pre></div>
</div>
<p>These can also be set via a configuration file:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">iscsi</span><span class="p">]</span>
<span class="n">user</span> <span class="o">=</span> <span class="s2">&quot;CHAP username&quot;</span>
<span class="n">password</span> <span class="o">=</span> <span class="s2">&quot;CHAP password&quot;</span>
<span class="n">initiator</span><span class="o">-</span><span class="n">name</span> <span class="o">=</span> <span class="s2">&quot;iqn.qemu.test:my-initiator&quot;</span>
<span class="c1"># header digest is one of CRC32C|CRC32C-NONE|NONE-CRC32C|NONE</span>
<span class="n">header</span><span class="o">-</span><span class="n">digest</span> <span class="o">=</span> <span class="s2">&quot;CRC32C&quot;</span>
</pre></div>
</div>
<p>Setting the target name allows different options for different targets:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">iscsi</span> <span class="s2">&quot;iqn.target.name&quot;</span><span class="p">]</span>
<span class="n">user</span> <span class="o">=</span> <span class="s2">&quot;CHAP username&quot;</span>
<span class="n">password</span> <span class="o">=</span> <span class="s2">&quot;CHAP password&quot;</span>
<span class="n">initiator</span><span class="o">-</span><span class="n">name</span> <span class="o">=</span> <span class="s2">&quot;iqn.qemu.test:my-initiator&quot;</span>
<span class="c1"># header digest is one of CRC32C|CRC32C-NONE|NONE-CRC32C|NONE</span>
<span class="n">header</span><span class="o">-</span><span class="n">digest</span> <span class="o">=</span> <span class="s2">&quot;CRC32C&quot;</span>
</pre></div>
</div>
<p>How to use a configuration file to set iSCSI configuration options:</p>
<pre class="literal-block">cat &gt;iscsi.conf &lt;&lt;EOF
[iscsi]
user = &quot;me&quot;
password = &quot;my password&quot;
initiator-name = &quot;iqn.qemu.test:my-initiator&quot;
header-digest = &quot;CRC32C&quot;
EOF
qemu-system-x86_64 -drive file=iscsi://127.0.0.1/iqn.qemu.test/1 \
-readconfig iscsi.conf</pre>
<p>How to set up a simple iSCSI target on loopback and access it via QEMU:
this example shows how to set up an iSCSI target with one CDROM and one DISK
using the Linux STGT software target. This target is available on Red Hat based
systems as the package ‘scsi-target-utils’.</p>
<pre class="literal-block">tgtd --iscsi portal=127.0.0.1:3260
tgtadm --lld iscsi --op new --mode target --tid 1 -T iqn.qemu.test
tgtadm --lld iscsi --mode logicalunit --op new --tid 1 --lun 1 \
-b /IMAGES/disk.img --device-type=disk
tgtadm --lld iscsi --mode logicalunit --op new --tid 1 --lun 2 \
-b /IMAGES/cd.iso --device-type=cd
tgtadm --lld iscsi --op bind --mode target --tid 1 -I ALL
qemu-system-x86_64 -iscsi initiator-name=iqn.qemu.test:my-initiator \
-boot d -drive file=iscsi://127.0.0.1/iqn.qemu.test/1 \
-cdrom iscsi://127.0.0.1/iqn.qemu.test/2</pre>
</section>
<section id="glusterfs-disk-images">
<h2>GlusterFS disk images<a class="headerlink" href="#glusterfs-disk-images" title="Link to this heading"></a></h2>
<p>GlusterFS is a user space distributed file system.</p>
<p>You can boot from the GlusterFS disk image with the command:</p>
<p>URI:</p>
<pre class="literal-block">qemu-system-x86_64 -drive file=gluster[+TYPE]://[HOST}[:PORT]]/VOLUME/PATH
[?socket=...][,file.debug=9][,file.logfile=...]</pre>
<p>JSON:</p>
<pre class="literal-block">qemu-system-x86_64 'json:{&quot;driver&quot;:&quot;qcow2&quot;,
&quot;file&quot;:{&quot;driver&quot;:&quot;gluster&quot;,
&quot;volume&quot;:&quot;testvol&quot;,&quot;path&quot;:&quot;a.img&quot;,&quot;debug&quot;:9,&quot;logfile&quot;:&quot;...&quot;,
&quot;server&quot;:[{&quot;type&quot;:&quot;tcp&quot;,&quot;host&quot;:&quot;...&quot;,&quot;port&quot;:&quot;...&quot;},
{&quot;type&quot;:&quot;unix&quot;,&quot;socket&quot;:&quot;...&quot;}]}}'</pre>
<p><em>gluster</em> is the protocol.</p>
<p><em>TYPE</em> specifies the transport type used to connect to gluster
management daemon (glusterd). Valid transport types are
tcp and unix. In the URI form, if a transport type isn’t specified,
then tcp type is assumed.</p>
<p><em>HOST</em> specifies the server where the volume file specification for
the given volume resides. This can be either a hostname or an ipv4 address.
If transport type is unix, then <em>HOST</em> field should not be specified.
Instead <em>socket</em> field needs to be populated with the path to unix domain
socket.</p>
<p><em>PORT</em> is the port number on which glusterd is listening. This is optional
and if not specified, it defaults to port 24007. If the transport type is unix,
then <em>PORT</em> should not be specified.</p>
<p><em>VOLUME</em> is the name of the gluster volume which contains the disk image.</p>
<p><em>PATH</em> is the path to the actual disk image that resides on gluster volume.</p>
<p><em>debug</em> is the logging level of the gluster protocol driver. Debug levels
are 0-9, with 9 being the most verbose, and 0 representing no debugging output.
The default level is 4. The current logging levels defined in the gluster source
are 0 - None, 1 - Emergency, 2 - Alert, 3 - Critical, 4 - Error, 5 - Warning,
6 - Notice, 7 - Info, 8 - Debug, 9 - Trace</p>
<p><em>logfile</em> is a commandline option to mention log file path which helps in
logging to the specified file and also help in persisting the gfapi logs. The
default is stderr.</p>
<p>You can create a GlusterFS disk image with the command:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">qemu</span><span class="o">-</span><span class="n">img</span> <span class="n">create</span> <span class="n">gluster</span><span class="p">:</span><span class="o">//</span><span class="n">HOST</span><span class="o">/</span><span class="n">VOLUME</span><span class="o">/</span><span class="n">PATH</span> <span class="n">SIZE</span>
</pre></div>
</div>
<p>Examples</p>
<pre class="literal-block">qemu-system-x86_64 -drive file=gluster://1.2.3.4/testvol/a.img
qemu-system-x86_64 -drive file=gluster+tcp://1.2.3.4/testvol/a.img
qemu-system-x86_64 -drive file=gluster+tcp://1.2.3.4:24007/testvol/dir/a.img
qemu-system-x86_64 -drive file=gluster+tcp://[1:2:3:4:5:6:7:8]/testvol/dir/a.img
qemu-system-x86_64 -drive file=gluster+tcp://[1:2:3:4:5:6:7:8]:24007/testvol/dir/a.img
qemu-system-x86_64 -drive file=gluster+tcp://server.domain.com:24007/testvol/dir/a.img
qemu-system-x86_64 -drive file=gluster+unix:///testvol/dir/a.img?socket=/tmp/glusterd.socket
qemu-system-x86_64 -drive file=gluster+rdma://1.2.3.4:24007/testvol/a.img
qemu-system-x86_64 -drive file=gluster://1.2.3.4/testvol/a.img,file.debug=9,file.logfile=/var/log/qemu-gluster.log
qemu-system-x86_64 'json:{&quot;driver&quot;:&quot;qcow2&quot;,
&quot;file&quot;:{&quot;driver&quot;:&quot;gluster&quot;,
&quot;volume&quot;:&quot;testvol&quot;,&quot;path&quot;:&quot;a.img&quot;,
&quot;debug&quot;:9,&quot;logfile&quot;:&quot;/var/log/qemu-gluster.log&quot;,
&quot;server&quot;:[{&quot;type&quot;:&quot;tcp&quot;,&quot;host&quot;:&quot;1.2.3.4&quot;,&quot;port&quot;:24007},
{&quot;type&quot;:&quot;unix&quot;,&quot;socket&quot;:&quot;/var/run/glusterd.socket&quot;}]}}'
qemu-system-x86_64 -drive driver=qcow2,file.driver=gluster,file.volume=testvol,file.path=/path/a.img,
file.debug=9,file.logfile=/var/log/qemu-gluster.log,
file.server.0.type=tcp,file.server.0.host=1.2.3.4,file.server.0.port=24007,
file.server.1.type=unix,file.server.1.socket=/var/run/glusterd.socket</pre>
</section>
<section id="secure-shell-ssh-disk-images">
<h2>Secure Shell (ssh) disk images<a class="headerlink" href="#secure-shell-ssh-disk-images" title="Link to this heading"></a></h2>
<p>You can access disk images located on a remote ssh server
by using the ssh protocol:</p>
<pre class="literal-block">qemu-system-x86_64 -drive file=ssh://[USER&#64;]SERVER[:PORT]/PATH[?host_key_check=HOST_KEY_CHECK]</pre>
<p>Alternative syntax using properties:</p>
<pre class="literal-block">qemu-system-x86_64 -drive file.driver=ssh[,file.user=USER],file.host=SERVER[,file.port=PORT],file.path=PATH[,file.host_key_check=HOST_KEY_CHECK]</pre>
<p><em>ssh</em> is the protocol.</p>
<p><em>USER</em> is the remote user. If not specified, then the local
username is tried.</p>
<p><em>SERVER</em> specifies the remote ssh server. Any ssh server can be
used, but it must implement the sftp-server protocol. Most Unix/Linux
systems should work without requiring any extra configuration.</p>
<p><em>PORT</em> is the port number on which sshd is listening. By default
the standard ssh port (22) is used.</p>
<p><em>PATH</em> is the path to the disk image.</p>
<p>The optional <em>HOST_KEY_CHECK</em> parameter controls how the remote
host’s key is checked. The default is <code class="docutils literal notranslate"><span class="pre">yes</span></code> which means to use
the local <code class="docutils literal notranslate"><span class="pre">.ssh/known_hosts</span></code> file. Setting this to <code class="docutils literal notranslate"><span class="pre">no</span></code>
turns off known-hosts checking. Or you can check that the host key
matches a specific fingerprint. The fingerprint can be provided in
<code class="docutils literal notranslate"><span class="pre">md5</span></code>, <code class="docutils literal notranslate"><span class="pre">sha1</span></code>, or <code class="docutils literal notranslate"><span class="pre">sha256</span></code> format, however, it is strongly
recommended to only use <code class="docutils literal notranslate"><span class="pre">sha256</span></code>, since the other options are
considered insecure by modern standards. The fingerprint value
must be given as a hex encoded string:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">host_key_check</span><span class="o">=</span><span class="n">sha256</span><span class="p">:</span><span class="mi">04</span><span class="n">ce2ae89ff4295a6b9c4111640bdcb3297858ee55cb434d9dd88796e93aa795</span>
</pre></div>
</div>
<p>The key string may optionally contain “:” separators between
each pair of hex digits.</p>
<p>The <code class="docutils literal notranslate"><span class="pre">$HOME/.ssh/known_hosts</span></code> file contains the base64 encoded
host keys. These can be converted into the format needed for
QEMU using a command such as:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>$ for key in `grep 10.33.8.112 known_hosts | awk &#39;{print $3}&#39;`
do
echo $key | base64 -d | sha256sum
done
6c3aa525beda9dc83eadfbd7e5ba7d976ecb59575d1633c87cd06ed2ed6e366f -
12214fd9ea5b408086f98ecccd9958609bd9ac7c0ea316734006bc7818b45dc8 -
d36420137bcbd101209ef70c3b15dc07362fbe0fa53c5b135eba6e6afa82f0ce -
</pre></div>
</div>
<p>Note that there can be multiple keys present per host, each with
different key ciphers. Care is needed to pick the key fingerprint
that matches the cipher QEMU will negotiate with the remote server.</p>
<p>Currently authentication must be done using ssh-agent. Other
authentication methods may be supported in future.</p>
<p>Note: Many ssh servers do not support an <code class="docutils literal notranslate"><span class="pre">fsync</span></code>-style operation.
The ssh driver cannot guarantee that disk flush requests are
obeyed, and this causes a risk of disk corruption if the remote
server or network goes down during writes. The driver will
print a warning when <code class="docutils literal notranslate"><span class="pre">fsync</span></code> is not supported:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">warning</span><span class="p">:</span> <span class="n">ssh</span> <span class="n">server</span> <span class="n">ssh</span><span class="o">.</span><span class="n">example</span><span class="o">.</span><span class="n">com</span><span class="p">:</span><span class="mi">22</span> <span class="n">does</span> <span class="ow">not</span> <span class="n">support</span> <span class="n">fsync</span>
</pre></div>
</div>
<p>With sufficiently new versions of libssh and OpenSSH, <code class="docutils literal notranslate"><span class="pre">fsync</span></code> is
supported.</p>
</section>
<section id="nvme-disk-images">
<h2>NVMe disk images<a class="headerlink" href="#nvme-disk-images" title="Link to this heading"></a></h2>
<p>NVM Express (NVMe) storage controllers can be accessed directly by a userspace
driver in QEMU. This bypasses the host kernel file system and block layers
while retaining QEMU block layer functionalities, such as block jobs, I/O
throttling, image formats, etc. Disk I/O performance is typically higher than
with <code class="docutils literal notranslate"><span class="pre">-drive</span> <span class="pre">file=/dev/sda</span></code> using either thread pool or linux-aio.</p>
<p>The controller will be exclusively used by the QEMU process once started. To be
able to share storage between multiple VMs and other applications on the host,
please use the file based protocols.</p>
<p>Before starting QEMU, bind the host NVMe controller to the host vfio-pci
driver. For example:</p>
<pre class="literal-block"># modprobe vfio-pci
# lspci -n -s 0000:06:0d.0
06:0d.0 0401: 1102:0002 (rev 08)
# echo 0000:06:0d.0 &gt; /sys/bus/pci/devices/0000:06:0d.0/driver/unbind
# echo 1102 0002 &gt; /sys/bus/pci/drivers/vfio-pci/new_id
# qemu-system-x86_64 -drive file=nvme://HOST:BUS:SLOT.FUNC/NAMESPACE</pre>
<p>Alternative syntax using properties:</p>
<pre class="literal-block">qemu-system-x86_64 -drive file.driver=nvme,file.device=HOST:BUS:SLOT.FUNC,file.namespace=NAMESPACE</pre>
<p><em>HOST</em>:<em>BUS</em>:<em>SLOT</em>.<em>FUNC</em> is the NVMe controller’s PCI device
address on the host.</p>
<p><em>NAMESPACE</em> is the NVMe namespace number, starting from 1.</p>
</section>
<section id="disk-image-file-locking">
<h2>Disk image file locking<a class="headerlink" href="#disk-image-file-locking" title="Link to this heading"></a></h2>
<p>By default, QEMU tries to protect image files from unexpected concurrent
access, as long as it’s supported by the block protocol driver and host
operating system. If multiple QEMU processes (including QEMU emulators and
utilities) try to open the same image with conflicting accessing modes, all but
the first one will get an error.</p>
<p>This feature is currently supported by the file protocol on Linux with the Open
File Descriptor (OFD) locking API, and can be configured to fall back to POSIX
locking if the POSIX host doesn’t support Linux OFD locking.</p>
<p>To explicitly enable image locking, specify “locking=on” in the file protocol
driver options. If OFD locking is not possible, a warning will be printed and
the POSIX locking API will be used. In this case there is a risk that the lock
will get silently lost when doing hot plugging and block jobs, due to the
shortcomings of the POSIX locking API.</p>
<p>QEMU transparently handles lock handover during shared storage migration. For
shared virtual disk images between multiple VMs, the “share-rw” device option
should be used.</p>
<p>By default, the guest has exclusive write access to its disk image. If the
guest can safely share the disk image with other writers the
<code class="docutils literal notranslate"><span class="pre">-device</span> <span class="pre">...,share-rw=on</span></code> parameter can be used. This is only safe if
the guest is running software, such as a cluster file system, that
coordinates disk accesses to avoid corruption.</p>
<p>Note that share-rw=on only declares the guest’s ability to share the disk.
Some QEMU features, such as image file formats, require exclusive write access
to the disk image and this is unaffected by the share-rw=on option.</p>
<p>Alternatively, locking can be fully disabled by “locking=off” block device
option. In the command line, the option is usually in the form of
“file.locking=off” as the protocol driver is normally placed as a “file” child
under a format driver. For example:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">-</span><span class="n">blockdev</span> <span class="n">driver</span><span class="o">=</span><span class="n">qcow2</span><span class="p">,</span><span class="n">file</span><span class="o">.</span><span class="n">filename</span><span class="o">=/</span><span class="n">path</span><span class="o">/</span><span class="n">to</span><span class="o">/</span><span class="n">image</span><span class="p">,</span><span class="n">file</span><span class="o">.</span><span class="n">locking</span><span class="o">=</span><span class="n">off</span><span class="p">,</span><span class="n">file</span><span class="o">.</span><span class="n">driver</span><span class="o">=</span><span class="n">file</span>
</pre></div>
</div>
<p>To check if image locking is active, check the output of the “lslocks” command
on host and see if there are locks held by the QEMU process on the image file.
More than one byte could be locked by the QEMU instance, each byte of which
reflects a particular permission that is acquired or protected by the running
block driver.</p>
</section>
<section id="filter-drivers">
<h2>Filter drivers<a class="headerlink" href="#filter-drivers" title="Link to this heading"></a></h2>
<p>QEMU supports several filter drivers, which don’t store any data, but perform
some additional tasks, hooking io requests.</p>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-filter-drivers-arg-preallocate">
<span class="sig-name descname"><span class="pre">preallocate</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-filter-drivers-arg-preallocate" title="Link to this definition"></a></dt>
<dd><p>The preallocate filter driver is intended to be inserted between format
and protocol nodes and preallocates some additional space
(expanding the protocol file) when writing past the file’s end. This can be
useful for file-systems with slow allocation.</p>
<p>Supported options:</p>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-preallocate-arg-prealloc-align">
<span class="sig-name descname"><span class="pre">prealloc-align</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-preallocate-arg-prealloc-align" title="Link to this definition"></a></dt>
<dd><p>On preallocation, align the file length to this value (in bytes), default 1M.</p>
</dd></dl>
<dl class="std option">
<dt class="sig sig-object std" id="cmdoption-preallocate-arg-prealloc-size">
<span class="sig-name descname"><span class="pre">prealloc-size</span></span><span class="sig-prename descclassname"></span><a class="headerlink" href="#cmdoption-preallocate-arg-prealloc-size" title="Link to this definition"></a></dt>
<dd><p>How much to preallocate (in bytes), default 128M.</p>
</dd></dl>
</dd></dl>
</section>
</section>
</div>
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