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  <div class="section" id="conversion-between-disk-templates">
<h1><a class="toc-backref" href="#id1">Conversion between disk templates</a><a class="headerlink" href="#conversion-between-disk-templates" 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">2014-May-23</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">2.13.0</td>
</tr>
</tbody>
</table>
<div class="contents topic" id="contents">
<p class="topic-title first">Contents</p>
<ul class="simple">
<li><a class="reference internal" href="#conversion-between-disk-templates" id="id1">Conversion between disk templates</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="#design-decisions" id="id4">Design decisions</a><ul>
<li><a class="reference internal" href="#block-device-changes" id="id5">Block device changes</a></li>
<li><a class="reference internal" href="#backend-changes" id="id6">Backend changes</a></li>
<li><a class="reference internal" href="#core-changes" id="id7">Core changes</a></li>
<li><a class="reference internal" href="#user-interface-changes" id="id8">User interface changes</a><ul>
<li><a class="reference internal" href="#extstorage-conversions" id="id9">ExtStorage conversions</a></li>
<li><a class="reference internal" href="#file-based-conversions" id="id10">File-based conversions</a></li>
</ul>
</li>
</ul>
</li>
<li><a class="reference internal" href="#supported-template-conversions" id="id11">Supported template conversions</a></li>
<li><a class="reference internal" href="#future-work" id="id12">Future Work</a></li>
</ul>
</li>
</ul>
</div>
<p>This design document describes the support for generic disk template
conversion in Ganeti. The logic used is disk template agnostic and
targets to cover the majority of conversions among the supported disk
templates.</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>Currently, Ganeti supports choosing among different disk templates when
creating an instance. However, converting the disk template of an
existing instance is possible only between the <code class="docutils literal"><span class="pre">plain</span></code> and <code class="docutils literal"><span class="pre">drbd</span></code>
templates. This feature was added in Ganeti since its early versions
when the number of supported disk templates was limited. Now that Ganeti
supports plenty of choices, this feature should be extended to provide
more flexibility to the user.</p>
<p>The procedure for converting from the plain to the drbd disk template
works as follows. Firstly, a completely new disk template is generated
matching the size, mode, and the count of the current instance’s disks.
The missing volumes are created manually both in the primary (meta disk)
and the secondary node. The original LVs running on the primary node are
renamed to match the new names. The last step is to manually associate
the DRBD devices with their mirror block device pairs. The conversion
from the drbd to the plain disk template is much simpler than the
opposite. Firstly, the DRBD mirroring is manually disabled. Then the
unnecessary volumes including the meta disk(s) of the primary node, and
the meta and data disk(s) from the previously secondary node are
removed.</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>This design proposes the creation of a unified interface for handling
the disk template conversions in Ganeti. Currently, there is no such
interface and each one of the supported conversions uses a separate code
path.</p>
<p>This proposal introduces a single, disk-agnostic interface for handling
the disk template conversions in Ganeti, keeping in mind that we want it
to be as generic as possible. An exception case will be the currently
supported conversions between the LVM-based disk templates. Their basic
functionality will not be affected and will diverge from the rest disk
template conversions. The target is to provide support for conversions
among the majority of the available disk templates, and also creating
a mechanism that will easily support any new templates that may be
probably added in Ganeti, at a future point.</p>
</div>
<div class="section" id="design-decisions">
<h2><a class="toc-backref" href="#id4">Design decisions</a><a class="headerlink" href="#design-decisions" title="Permalink to this headline"></a></h2>
<p>Currently, the supported conversions for the LVM-based templates are
handled by the <code class="docutils literal"><span class="pre">LUInstanceSetParams</span></code> LU. Our implementation will
follow the same approach. From a high-level point-of-view this design
can be split in two parts:</p>
<ul class="simple">
<li>The extension of the LU’s checks to cover all the supported template
conversions</li>
<li>The new functionality which will be introduced to provide the new
feature</li>
</ul>
<p>The instance must be stopped before starting the disk template
conversion, as it currently is, otherwise the operation will fail. The
new mechanism will need to copy the disk’s data for the conversion to be
possible. We propose using the Unix <code class="docutils literal"><span class="pre">dd</span></code> command to copy the
instance’s data. It can be used to copy data from source to destination,
block-by-block, regardless of their filesystem types, making it a
convenient tool for the case. Since the conversion will be done via data
copy it will take a long time for bigger disks to copy their data and
consequently for the instance to switch to the new template.</p>
<p>Some template conversions can be done faster without copying explicitly
their disks’ data. A use case is the conversions between the LVM-based
templates, i.e., <code class="docutils literal"><span class="pre">drbd</span></code> and <code class="docutils literal"><span class="pre">plain</span></code> which will be done as happens
now and not using the <code class="docutils literal"><span class="pre">dd</span></code> command. Also, this implementation will
provide partial support for the <code class="docutils literal"><span class="pre">blockdev</span></code> disk template which will
act only as a source template. Since those volumes are adopted
pre-existent block devices we will not support conversions targeting
this template. Another exception case will be the <code class="docutils literal"><span class="pre">diskless</span></code> template.
Since it is a testing template that creates instances with no disks we
will not provide support for conversions that include this template
type.</p>
<p>We divide the design into the following parts:</p>
<ul class="simple">
<li>Block device changes, that include the new methods which will be
introduced and will be responsible for building the commands for the
data copy from/to the requested devices</li>
<li>Backend changes, that include a new RPC call which will concatenate
the output of the above two methods and will execute the data copy
command</li>
<li>Core changes, that include the modifications in the Logical Unit</li>
<li>User interface changes, i.e., command line changes</li>
</ul>
<div class="section" id="block-device-changes">
<h3><a class="toc-backref" href="#id5">Block device changes</a><a class="headerlink" href="#block-device-changes" title="Permalink to this headline"></a></h3>
<p>The block device abstract class will be extended with two new methods,
named <code class="docutils literal"><span class="pre">Import</span></code> and <code class="docutils literal"><span class="pre">Export</span></code>. Those methods will be responsible for
building the commands that will be used for the data copy between the
corresponding devices. The <code class="docutils literal"><span class="pre">Export</span></code> method will build the command
which will export the data from the source device, while the <code class="docutils literal"><span class="pre">Import</span></code>
method will do the opposite. It will import the data to the newly
created target device. Those two methods will not perform the actual
data copy; they will simply return the requested commands for
transferring the data from/to the individual devices. The output of the
two methods will be combined using a pipe (“|”) by the caller method in
the backend level.</p>
<p>By default the data import and export will be done using the <code class="docutils literal"><span class="pre">dd</span></code>
command. All the inherited classes will use the base functionality
unless there is a faster way to convert to. In that case the underlying
block device will overwrite those methods with its specific
functionality. A use case will be the Ceph/RADOS block devices which
will make use of the <code class="docutils literal"><span class="pre">rbd</span> <span class="pre">import</span></code> and <code class="docutils literal"><span class="pre">rbd</span> <span class="pre">export</span></code> commands to copy
their data instead of using the default <code class="docutils literal"><span class="pre">dd</span></code> command.</p>
<p>Keeping the data copy functionality in the block device layer, provides
us with a generic mechanism that works between almost all conversions
and furthermore can be easily extended for new disk templates. It also
covers the devices that support the <code class="docutils literal"><span class="pre">access=userspace</span></code> parameter and
solves this problem in a generic way, by implementing the logic in the
right level where we know what is the best to do for each device.</p>
</div>
<div class="section" id="backend-changes">
<h3><a class="toc-backref" href="#id6">Backend changes</a><a class="headerlink" href="#backend-changes" title="Permalink to this headline"></a></h3>
<p>Introduce a new RPC call:</p>
<ul class="simple">
<li>blockdev_convert(src_disk, dest_disk)</li>
</ul>
<p>where <code class="docutils literal"><span class="pre">src_disk</span></code> and <code class="docutils literal"><span class="pre">dest_disk</span></code> are the original and the new disk
objects respectively. First, the actual device instances will be
computed and then they will be used to build the export and import
commands for the data copy. The output of those methods will be
concatenated using a pipe, following a similar approach with the impexp
daemon. Finally, the unified data copy command will be executed, at this
level, by the <code class="docutils literal"><span class="pre">nodeD</span></code>.</p>
</div>
<div class="section" id="core-changes">
<h3><a class="toc-backref" href="#id7">Core changes</a><a class="headerlink" href="#core-changes" title="Permalink to this headline"></a></h3>
<p>The main modifications will be made in the <code class="docutils literal"><span class="pre">LUInstanceSetParams</span></code> LU.
The implementation of the conversion mechanism will be split into the
following parts:</p>
<ul class="simple">
<li>The generation of the new disk template for the instance. The new
disks will match the size, mode, and name of the original volumes.
Those parameters and any other needed, .i.e., the provider’s name for
the ExtStorage conversions, will be computed by a new method which we
will introduce, named <code class="docutils literal"><span class="pre">ComputeDisksInfo</span></code>. The output of that
function will be used as the <code class="docutils literal"><span class="pre">disk_info</span></code> argument of the
<code class="docutils literal"><span class="pre">GenerateDiskTemplate</span></code> method.</li>
<li>The creation of the new block devices. We will make use of the
<code class="docutils literal"><span class="pre">CreateDisks</span></code> method which creates and attaches the new block
devices.</li>
<li>The data copy for each disk of the instance from the original to the
newly created volume. The data copy will be made by the <code class="docutils literal"><span class="pre">nodeD</span></code> with
the rpc call we have introduced earlier in this design. In case some
disks fail to copy their data the operation will fail and the newly
created disks will be removed. The instance will remain intact.</li>
<li>The detachment of the original disks of the instance when the data
copy operation successfully completes by calling the
<code class="docutils literal"><span class="pre">RemoveInstanceDisk</span></code> method for each instance’s disk.</li>
<li>The attachment of the new disks to the instance by calling the
<code class="docutils literal"><span class="pre">AddInstanceDisk</span></code> method for each disk we have created.</li>
<li>The update of the configuration file with the new values.</li>
<li>The removal of the original block devices from the node using the
<code class="docutils literal"><span class="pre">BlockdevRemove</span></code> method for each one of the old disks.</li>
</ul>
</div>
<div class="section" id="user-interface-changes">
<h3><a class="toc-backref" href="#id8">User interface changes</a><a class="headerlink" href="#user-interface-changes" title="Permalink to this headline"></a></h3>
<p>The <code class="docutils literal"><span class="pre">-t</span></code> (<code class="docutils literal"><span class="pre">--disk-template</span></code>) option from the gnt-instance modify
command will specify the disk template to convert <em>to</em>, as it happens
now. The rest disk options such as its size, its mode, and its name will
be computed from the original volumes by the conversion mechanism, and
the user will not explicitly provide them.</p>
<div class="section" id="extstorage-conversions">
<h4><a class="toc-backref" href="#id9">ExtStorage conversions</a><a class="headerlink" href="#extstorage-conversions" title="Permalink to this headline"></a></h4>
<p>When converting to an ExtStorage disk template the
<code class="docutils literal"><span class="pre">provider=*PROVIDER*</span></code> option which specifies the ExtStorage provider
will be mandatory. Also, arbitrary parameters can be passed to the
ExtStorage provider. Those parameters will be optional and could be
passed as additional comma separated options. Since it is not allowed to
convert the disk template of an instance and make use of the <code class="docutils literal"><span class="pre">--disk</span></code>
option at the same time, we propose to introduce a new option named
<code class="docutils literal"><span class="pre">--ext-params</span></code> to handle the <code class="docutils literal"><span class="pre">ext</span></code> template conversions.</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">gnt</span><span class="o">-</span><span class="n">instance</span> <span class="n">modify</span> <span class="o">-</span><span class="n">t</span> <span class="n">ext</span> <span class="o">--</span><span class="n">ext</span><span class="o">-</span><span class="n">params</span> <span class="n">provider</span><span class="o">=</span><span class="n">pvdr1</span> <span class="n">test_vm</span>
<span class="n">gnt</span><span class="o">-</span><span class="n">instance</span> <span class="n">modify</span> <span class="o">-</span><span class="n">t</span> <span class="n">ext</span> <span class="o">--</span><span class="n">ext</span><span class="o">-</span><span class="n">params</span> <span class="n">provider</span><span class="o">=</span><span class="n">pvdr1</span><span class="p">,</span><span class="n">param1</span><span class="o">=</span><span class="n">val1</span><span class="p">,</span><span class="n">param2</span><span class="o">=</span><span class="n">val2</span> <span class="n">test_vm</span>
</pre></div>
</div>
</div>
<div class="section" id="file-based-conversions">
<h4><a class="toc-backref" href="#id10">File-based conversions</a><a class="headerlink" href="#file-based-conversions" title="Permalink to this headline"></a></h4>
<p>For conversions <em>to</em> a file-based template the <code class="docutils literal"><span class="pre">--file-storage-dir</span></code>
and the <code class="docutils literal"><span class="pre">--file-driver</span></code> options could be used, similarly to the
<strong>add</strong> command, to manually configure the storage directory and the
preferred driver for the file-based disks.</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">gnt</span><span class="o">-</span><span class="n">instance</span> <span class="n">modify</span> <span class="o">-</span><span class="n">t</span> <span class="n">file</span> <span class="o">--</span><span class="n">file</span><span class="o">-</span><span class="n">storage</span><span class="o">-</span><span class="nb">dir</span><span class="o">=</span><span class="n">mysubdir</span> <span class="n">test_vm</span>
</pre></div>
</div>
</div>
</div>
</div>
<div class="section" id="supported-template-conversions">
<h2><a class="toc-backref" href="#id11">Supported template conversions</a><a class="headerlink" href="#supported-template-conversions" title="Permalink to this headline"></a></h2>
<p>This is a summary of the disk template conversions that the conversion
mechanism will support:</p>
<table border="1" class="docutils">
<colgroup>
<col width="16%" />
<col width="10%" />
<col width="8%" />
<col width="7%" />
<col width="13%" />
<col width="10%" />
<col width="7%" />
<col width="7%" />
<col width="11%" />
<col width="11%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head" rowspan="2">Source
Disk
Template</th>
<th class="head" colspan="9">Target Disk Template</th>
</tr>
<tr class="row-even"><th class="head">Plain</th>
<th class="head">DRBD</th>
<th class="head">File</th>
<th class="head">Sharedfile</th>
<th class="head">Gluster</th>
<th class="head">RBD</th>
<th class="head">Ext</th>
<th class="head">BlockDev</th>
<th class="head">Diskless</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-odd"><td>Plain</td>
<td><ul class="first last simple">
<li></li>
</ul>
</td>
<td>Yes.</td>
<td>Yes.</td>
<td>Yes.</td>
<td>Yes.</td>
<td>Yes.</td>
<td>Yes.</td>
<td>No.</td>
<td>No.</td>
</tr>
<tr class="row-even"><td>DRBD</td>
<td>Yes.</td>
<td><ul class="first last simple">
<li></li>
</ul>
</td>
<td>Yes.</td>
<td>Yes.</td>
<td>Yes.</td>
<td>Yes.</td>
<td>Yes.</td>
<td>No.</td>
<td>No.</td>
</tr>
<tr class="row-odd"><td>File</td>
<td>Yes.</td>
<td>Yes.</td>
<td><ul class="first last simple">
<li></li>
</ul>
</td>
<td>Yes.</td>
<td>Yes.</td>
<td>Yes.</td>
<td>Yes.</td>
<td>No.</td>
<td>No.</td>
</tr>
<tr class="row-even"><td>Sharedfile</td>
<td>Yes.</td>
<td>Yes.</td>
<td>Yes.</td>
<td><ul class="first last simple">
<li></li>
</ul>
</td>
<td>Yes.</td>
<td>Yes.</td>
<td>Yes.</td>
<td>No.</td>
<td>No.</td>
</tr>
<tr class="row-odd"><td>Gluster</td>
<td>Yes.</td>
<td>Yes.</td>
<td>Yes.</td>
<td>Yes.</td>
<td><ul class="first last simple">
<li></li>
</ul>
</td>
<td>Yes.</td>
<td>Yes.</td>
<td>No.</td>
<td>No.</td>
</tr>
<tr class="row-even"><td>RBD</td>
<td>Yes.</td>
<td>Yes.</td>
<td>Yes.</td>
<td>Yes.</td>
<td>Yes.</td>
<td><ul class="first last simple">
<li></li>
</ul>
</td>
<td>Yes.</td>
<td>No.</td>
<td>No.</td>
</tr>
<tr class="row-odd"><td>Ext</td>
<td>Yes.</td>
<td>Yes.</td>
<td>Yes.</td>
<td>Yes.</td>
<td>Yes.</td>
<td>Yes.</td>
<td><ul class="first last simple">
<li></li>
</ul>
</td>
<td>No.</td>
<td>No.</td>
</tr>
<tr class="row-even"><td>BlockDev</td>
<td>Yes.</td>
<td>Yes.</td>
<td>Yes.</td>
<td>Yes.</td>
<td>Yes.</td>
<td>Yes.</td>
<td>Yes.</td>
<td><ul class="first last simple">
<li></li>
</ul>
</td>
<td>No.</td>
</tr>
<tr class="row-odd"><td>Diskless</td>
<td>No.</td>
<td>No.</td>
<td>No.</td>
<td>No.</td>
<td>No.</td>
<td>No.</td>
<td>No.</td>
<td>No.</td>
<td><ul class="first last simple">
<li></li>
</ul>
</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="future-work">
<h2><a class="toc-backref" href="#id12">Future Work</a><a class="headerlink" href="#future-work" title="Permalink to this headline"></a></h2>
<p>Expand the conversion mechanism to provide a visual indication of the
data copy operation. We could monitor the progress of the data sent via
a pipe, and provide to the user information such as the time elapsed,
percentage completed (probably with a progress bar), total data
transferred, and so on, similar to the progress tracking that is
currently done by the impexp daemon.</p>
</div>
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  <h3><a href="index.html">Table Of Contents</a></h3>
  <ul>
<li><a class="reference internal" href="#">Conversion between disk templates</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="#design-decisions">Design decisions</a><ul>
<li><a class="reference internal" href="#block-device-changes">Block device changes</a></li>
<li><a class="reference internal" href="#backend-changes">Backend changes</a></li>
<li><a class="reference internal" href="#core-changes">Core changes</a></li>
<li><a class="reference internal" href="#user-interface-changes">User interface changes</a><ul>
<li><a class="reference internal" href="#extstorage-conversions">ExtStorage conversions</a></li>
<li><a class="reference internal" href="#file-based-conversions">File-based conversions</a></li>
</ul>
</li>
</ul>
</li>
<li><a class="reference internal" href="#supported-template-conversions">Supported template conversions</a></li>
<li><a class="reference internal" href="#future-work">Future Work</a></li>
</ul>
</li>
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