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<div class="section" id="ganeti-file-based-disks-ownership">
<h1><a class="toc-backref" href="#id1">Ganeti file-based disks ownership</a><a class="headerlink" href="#ganeti-file-based-disks-ownership" title="Permalink to this headline">¶</a></h1>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field-odd field"><th class="field-name">Created:</th><td class="field-body">2015-Jan-20</td>
</tr>
<tr class="field-even field"><th class="field-name">Status:</th><td class="field-body">Implemented</td>
</tr>
<tr class="field-odd field"><th class="field-name">Ganeti-Version:</th><td class="field-body">2.14.0</td>
</tr>
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<div class="contents topic" id="contents">
<p class="topic-title first">Contents</p>
<ul class="simple">
<li><a class="reference internal" href="#ganeti-file-based-disks-ownership" id="id1">Ganeti file-based disks ownership</a><ul>
<li><a class="reference internal" href="#current-state-and-shortcomings" id="id2">Current state and shortcomings</a></li>
<li><a class="reference internal" href="#proposed-changes" id="id3">Proposed changes</a></li>
<li><a class="reference internal" href="#implementation-details" id="id4">Implementation details</a></li>
</ul>
</li>
</ul>
</div>
<p>This design document explains the issue that emerges from the usage of the
<cite>detach</cite> operation to file-based disks and provides a simple solution to it.
Note that this design document applies only to disks of template <cite>file</cite> and
<cite>sharedfile</cite>, but not <cite>gluster</cite>. However, for brevity reasons these templates
will go under the umbrella term <cite>file-based</cite>.</p>
<div class="section" id="current-state-and-shortcomings">
<h2><a class="toc-backref" href="#id2">Current state and shortcomings</a><a class="headerlink" href="#current-state-and-shortcomings" title="Permalink to this headline">¶</a></h2>
<p>When creating a file-based disk, Ganeti stores it inside a specific directory,
called <cite>file_storage_dir</cite>. Inside this directory, there is a folder for each
file-based instance and inside each folder are the files for the instance’s
disks (e.g. <code class="docutils literal"><span class="pre"><file_storage_dir>/<instance_name>/<disk_name></span></code>). This way of
storing disks seems simple enough, but the
<cite>detach</cite> operation does not work well with it. The reason is that if a disk is
detached from an instance and attached to another one, the file will remain to
the folder of the original instance.</p>
<p>This means that if we try to destroy an instance with detached disks, Ganeti
will correctly complain that the instance folder still has disk data. In more
high-level terms, we need to find a way to resolve the issue of disk ownership
at the filesystem level for file-based instances.</p>
</div>
<div class="section" id="proposed-changes">
<h2><a class="toc-backref" href="#id3">Proposed changes</a><a class="headerlink" href="#proposed-changes" title="Permalink to this headline">¶</a></h2>
<p>The change we propose is simple. Once a disk is detached from an instance, it
will be moved out of the instance’s folder. The new location will be the
<cite>file_storage_dir</cite>, i.e. the disk will reside on the same level as the instance
folders. In order to maintain a consistent configuration, the logical_id of the
disk will be updated to point to the new path.</p>
<p>Similarly, on the <cite>attach</cite> operation, the file name and logical id will change
and the disk will be moved under the new instance’s directory.</p>
</div>
<div class="section" id="implementation-details">
<h2><a class="toc-backref" href="#id4">Implementation details</a><a class="headerlink" href="#implementation-details" title="Permalink to this headline">¶</a></h2>
<div class="section" id="detach-operation">
<h3>Detach operation<a class="headerlink" href="#detach-operation" title="Permalink to this headline">¶</a></h3>
<p>Before detaching a disk from an instance, we do the following:</p>
<ol class="arabic">
<li><p class="first">Transform the current path to the new one.</p>
<p><file_storage_dir>/<instance_name>/<disk_name> –> <file_storage_dir>/<disk_name></p>
</li>
<li><p class="first">Use the rpc call <code class="docutils literal"><span class="pre">call_blockdev_rename</span></code> to move the disk to the new path.</p>
</li>
<li><p class="first">Store the new <code class="docutils literal"><span class="pre">logical_id</span></code> to the configuration.</p>
</li>
</ol>
</div>
<div class="section" id="attach-operation">
<h3>Attach operation<a class="headerlink" href="#attach-operation" title="Permalink to this headline">¶</a></h3>
<p>Before attaching a disk to an instance, we do the following:</p>
<ol class="arabic simple">
<li>Create the new path for the file disk. In order to construct it properly,
use the <code class="docutils literal"><span class="pre">GenerateDiskTemplate</span></code> function to create a dummy disk template
and get its <code class="docutils literal"><span class="pre">logical_id</span></code>. The new <code class="docutils literal"><span class="pre">logical_id</span></code> contains the new path for
the file disk.</li>
<li>Use the rpc call <code class="docutils literal"><span class="pre">call_blockdev_rename</span></code> to move the disk to the new path.</li>
<li>Store the new <code class="docutils literal"><span class="pre">logical_id</span></code> to the configuration.</li>
</ol>
</div>
</div>
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<h3><a href="index.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">Ganeti file-based disks ownership</a><ul>
<li><a class="reference internal" href="#current-state-and-shortcomings">Current state and shortcomings</a></li>
<li><a class="reference internal" href="#proposed-changes">Proposed changes</a></li>
<li><a class="reference internal" href="#implementation-details">Implementation details</a><ul>
<li><a class="reference internal" href="#detach-operation">Detach operation</a></li>
<li><a class="reference internal" href="#attach-operation">Attach operation</a></li>
</ul>
</li>
</ul>
</li>
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<h4>Previous topic</h4>
<p class="topless"><a href="design-file-based-storage.html"
title="previous chapter">File-based Storage</a></p>
<h4>Next topic</h4>
<p class="topless"><a href="design-glusterfs-ganeti-support.html"
title="next chapter">GlusterFS Ganeti support</a></p>
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