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<section id="mailbox-distribution">
<span id="imap-features-mailbox-distribution"></span><h1>Mailbox Distribution<a class="headerlink" href="#mailbox-distribution" title="Permalink to this heading"></a></h1>
<p>Prior to Cyrus IMAP version 2.5.0, when creating a mailbox, should no
target partition have been specified, the mailbox is either created on:</p>
<ul class="simple">
<li><p>the configured <code class="docutils literal notranslate"><span class="pre">defaultserver</span></code>, or</p></li>
<li><p>the server that is found to have the most free disk space, and</p></li>
<li><p>the configured <code class="docutils literal notranslate"><span class="pre">defaultpartition</span></code>, or</p></li>
<li><p>the most fitting partition if <code class="docutils literal notranslate"><span class="pre">defaultpartition</span></code> is not
configured.</p></li>
</ul>
<p>New configuration options are available since Cyrus IMAP 2.5.0, that
allow more weighted and better balanced backend and partition
selection.</p>
<section id="partition-selection-mode">
<h2>Partition Selection Mode<a class="headerlink" href="#partition-selection-mode" title="Permalink to this heading"></a></h2>
<p>Among the partitions, how the most fitting one is selected depends on
the configured selection mode, using the <code class="docutils literal notranslate"><span class="pre">partition_select_mode</span></code>
setting in <a class="reference internal" href="../../reference/manpages/configs/imapd.conf.html#std-cyrusman-imapd.conf-5">imapd.conf(5)</a>.</p>
<p class="rubric">Available Selection Modes</p>
<aside class="sidebar">
<p class="sidebar-title">Example Scenario for Selection Modes</p>
<p>Suppose that the partitions configured are:</p>
<table class="docutils align-default">
<thead>
<tr class="row-odd"><th class="head"><p>Partition</p></th>
<th class="head"><p>Total</p></th>
<th class="head"><p>Free</p></th>
<th class="head"><p>%</p></th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">part1</span></code></p></td>
<td><p>1000 GB</p></td>
<td><p>400 GB</p></td>
<td><p>40%</p></td>
</tr>
<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">part2</span></code></p></td>
<td><p>1000 GB</p></td>
<td><p>600 GB</p></td>
<td><p>60%</p></td>
</tr>
<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">part3</span></code></p></td>
<td><p>100 GB</p></td>
<td><p>30 GB</p></td>
<td><p>30%</p></td>
</tr>
<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">part4</span></code></p></td>
<td><p>100 GB</p></td>
<td><p>70 GB</p></td>
<td><p>70%</p></td>
</tr>
</tbody>
</table>
</aside>
<ul>
<li><p><strong>random</strong></p>
<p>Choice is (pseudo-)random. Each partition has the same probability
of being selected.</p>
</li>
<li><p><strong>freespace-most</strong></p>
<p>The partition which has the most absolute free space (counted in KiB
units) is selected.</p>
<p>In the example scenario, <code class="docutils literal notranslate"><span class="pre">part2</span></code> would be selected as most
fitting, since 600 GB of free space is the biggest of all
partitions.</p>
</li>
<li><p><strong>freespace-percent-most</strong></p>
<p>The partition which has the most relative free space (counted in
percentiles) is selected.</p>
<p>In the example scenario, <code class="docutils literal notranslate"><span class="pre">part4</span></code> would be selected as most
fitting, since 70% of free space is the biggest of all partitions.</p>
</li>
<li><p><strong>freespace-percent-weighted</strong></p>
<p>For each partition, the percentage of free space is its weight. Then
a weighted choice is performed to select one of those partitions.</p>
<p>As such, the more free space the partition has, the higher its
chances to be selected.</p>
<p>In the example scenario, the weight of each partition would be:</p>
<ul class="simple">
<li><p>40 for <code class="docutils literal notranslate"><span class="pre">part1</span></code></p></li>
<li><p>60 for <code class="docutils literal notranslate"><span class="pre">part2</span></code></p></li>
<li><p>30 for <code class="docutils literal notranslate"><span class="pre">part3</span></code></p></li>
<li><p>70 for <code class="docutils literal notranslate"><span class="pre">part4</span></code></p></li>
</ul>
<p>The sum of all weights being 200, the probability for each
partition to be selected as most fitting is:</p>
<ul class="simple">
<li><p>20% for <code class="docutils literal notranslate"><span class="pre">part1</span></code></p></li>
<li><p>30% for <code class="docutils literal notranslate"><span class="pre">part2</span></code></p></li>
<li><p>15% for <code class="docutils literal notranslate"><span class="pre">part3</span></code></p></li>
<li><p>35% for <code class="docutils literal notranslate"><span class="pre">part4</span></code></p></li>
</ul>
<p>Out of 20 (hypothetically empty) mailboxes to be created,
chances are:</p>
<ul class="simple">
<li><p>4 are created on <code class="docutils literal notranslate"><span class="pre">part1</span></code></p></li>
<li><p>6 are created on <code class="docutils literal notranslate"><span class="pre">part2</span></code></p></li>
<li><p>3 are created on <code class="docutils literal notranslate"><span class="pre">part3</span></code></p></li>
<li><p>7 are created on <code class="docutils literal notranslate"><span class="pre">part4</span></code></p></li>
</ul>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>In <code class="docutils literal notranslate"><span class="pre">freespace-percent-weighted</span></code> mode, partitions percentage
usages converge towards 100%. So if they have different usages,
those differences will stay and only really diminish upon
reaching 100% of usage.</p>
<p>You may also observe growing differences between partitions
usage when those partitions do not have the same total disk
space.</p>
</div>
</li>
</ul>
<p><strong>freespace-percent-weighted-delta</strong></p>
<blockquote>
<div><p>As for <code class="docutils literal notranslate"><span class="pre">freespace-percent-weighted</span></code>, a weight is associated to
each partition. It is computed as follows:</p>
<div class="math notranslate nohighlight">
\[free - leastfree + 0.5\]</div>
<p>Then a weighted choice is performed to select one of those
partitions.</p>
<p>As such, considering the percentages of usage, the more the
partition is lagging behind the most used partition (which is the
one with the lowest <strong>percentage</strong> of free space), the higher its
chances are to be selected.</p>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>The added 0.5 in partitions weight causes the selection to get
smoother the more partitions get close to each other.</p>
</div>
<p>In the example scenario, the weight of each partition would be:</p>
<ul class="simple">
<li><p>For <code class="docutils literal notranslate"><span class="pre">part1</span></code>: <span class="math notranslate nohighlight">\(40 - 30 + 0.5 = 10.5\)</span></p></li>
<li><p>For <code class="docutils literal notranslate"><span class="pre">part2</span></code>: <span class="math notranslate nohighlight">\(60 - 30 + 0.5 = 30.5\)</span></p></li>
<li><p>For <code class="docutils literal notranslate"><span class="pre">part3</span></code>: <span class="math notranslate nohighlight">\(30 - 30 + 0.5 = 00.5\)</span></p></li>
<li><p>For <code class="docutils literal notranslate"><span class="pre">part4</span></code>: <span class="math notranslate nohighlight">\(70 - 30 + 0.5 = 40.5\)</span></p></li>
</ul>
<p>The sum of all weights amounting to 82, the probability for each
partition to be selected as most fitting would be:</p>
<ul class="simple">
<li><p>For <code class="docutils literal notranslate"><span class="pre">part1</span></code>: 12.8%</p></li>
<li><p>For <code class="docutils literal notranslate"><span class="pre">part2</span></code>: 37.2%</p></li>
<li><p>For <code class="docutils literal notranslate"><span class="pre">part3</span></code>: 00.6%</p></li>
<li><p>For <code class="docutils literal notranslate"><span class="pre">part4</span></code>: 49.4%</p></li>
</ul>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>In <code class="docutils literal notranslate"><span class="pre">freespace-percent-weighted-delta</span></code> mode, partitions
percentage usages converge towards the most used one, after
which partitions usages grow equally.</p>
</div>
</div></blockquote>
<section id="special-cases">
<h3>Special Cases<a class="headerlink" href="#special-cases" title="Permalink to this heading"></a></h3>
<section id="q-what-happens-when-two-partitions-are-equal-as-most-fitting">
<h4>Q: What happens when two partitions are equal as most fitting?<a class="headerlink" href="#q-what-happens-when-two-partitions-are-equal-as-most-fitting" title="Permalink to this heading"></a></h4>
<p>Suppose you are using the <code class="docutils literal notranslate"><span class="pre">freespace-most</span></code> selection mode, that two
(or more) partitions have the same free size, and that this freespace
happens to be the biggest one of all configured partitions.</p>
<p>In that case, only one of those partitions will be selected. You may not
know in advance which one will be: it depends of the order in which
configured partitions are stored in memory (<code class="docutils literal notranslate"><span class="pre">hashtable</span></code>). In
particular, it may not be the first one that appears listed in your
<a class="reference internal" href="../../reference/manpages/configs/imapd.conf.html#std-cyrusman-imapd.conf-5">imapd.conf(5)</a> configuration file.</p>
<p>Also note that since the selected partition will now have less free
space, it shall not be seen as most fitting next time.</p>
</section>
<section id="q-what-happens-when-two-partitions-point-to-the-same-device">
<h4>Q: What happens when two partitions point to the same device?<a class="headerlink" href="#q-what-happens-when-two-partitions-point-to-the-same-device" title="Permalink to this heading"></a></h4>
<p>Suppose you are using the <code class="docutils literal notranslate"><span class="pre">freespace-most</span></code> or
<code class="docutils literal notranslate"><span class="pre">freespace-percent-most</span></code> selection mode, and that two (or more)
partitions actually point to a different directory on the same
filesystem.</p>
<p>In that case, only one of those partitions will be checked, as if the
others were not configured. Cyrus IMAP uses the device id of the
partition hierarchy for this. You may not know in advance which one will
be: it depends of the order in which configured partitions are stored in
memory (<code class="docutils literal notranslate"><span class="pre">hashtable</span></code>). In particular, it may not be the first one that
appears listed in your <a class="reference internal" href="../../reference/manpages/configs/imapd.conf.html#std-cyrusman-imapd.conf-5">imapd.conf(5)</a> configuration file.</p>
</section>
</section>
<section id="excluding-partitions">
<h3>Excluding Partitions<a class="headerlink" href="#excluding-partitions" title="Permalink to this heading"></a></h3>
<p><strong>partition_select_exclude</strong></p>
<blockquote>
<div><p>Partitions listed in the <code class="docutils literal notranslate"><span class="pre">partition_select_exclude</span></code> setting in
<a class="reference internal" href="../../reference/manpages/configs/imapd.conf.html#std-cyrusman-imapd.conf-5">imapd.conf(5)</a> are permanently excluded from being
selected.</p>
<p>Listed partition's names are separated by spaces and/or commas. Only
configured partition names (and not paths) are expected.</p>
<p>With the following configuration in <a class="reference internal" href="../../reference/manpages/configs/imapd.conf.html#std-cyrusman-imapd.conf-5">imapd.conf(5)</a>,
<code class="docutils literal notranslate"><span class="pre">parta</span></code> and <code class="docutils literal notranslate"><span class="pre">partb</span></code> would be permanently excluded from being
selected:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">partition</span><span class="o">-</span><span class="n">part1</span><span class="p">:</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">part1</span>
<span class="n">partition</span><span class="o">-</span><span class="n">part2</span><span class="p">:</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">part2</span>
<span class="n">partition</span><span class="o">-</span><span class="n">part3</span><span class="p">:</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">part3</span>
<span class="n">partition</span><span class="o">-</span><span class="n">part4</span><span class="p">:</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">part4</span>
<span class="n">partition</span><span class="o">-</span><span class="n">parta</span><span class="p">:</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">parta</span>
<span class="n">partition</span><span class="o">-</span><span class="n">partb</span><span class="p">:</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">partb</span>
<span class="n">partition_mode_exclude</span><span class="p">:</span> <span class="n">parta</span> <span class="n">partb</span>
</pre></div>
</div>
</div></blockquote>
<p><strong>partition_select_soft_usage_limit</strong></p>
<blockquote>
<div><p>When using a selection mode other than <code class="docutils literal notranslate"><span class="pre">random</span></code>, partitions are
automatically excluded if their usage percentage is beyond the
<code class="docutils literal notranslate"><span class="pre">partition_select_soft_usage_limit</span></code> integer setting.</p>
<p>If all partitions are beyond the configured value, this feature is
automatically deactivated. A partition is thus selected as if the
setting was not configured.</p>
<p>In the example scenario, setting <code class="docutils literal notranslate"><span class="pre">partition_mode_soft_usage_limit</span></code>
to <code class="docutils literal notranslate"><span class="pre">50</span></code> would exclude partitions <code class="docutils literal notranslate"><span class="pre">part1</span></code> and <code class="docutils literal notranslate"><span class="pre">part3</span></code> since
their disk usages are 60% and 70% respectively.</p>
<p>But setting the option to <code class="docutils literal notranslate"><span class="pre">20</span></code> would have no immediate effect on
the selection of a partition, since the usage of all partitions is
beyond 20%.</p>
</div></blockquote>
<div class="admonition important">
<p class="admonition-title">Important</p>
<p><strong>Exclusion is not absolute</strong></p>
<p>Partitions are only excluded when creating a new user mailbox
according to the configured selection mode.</p>
<p>If you explicitly specify an excluded partition, the mailbox will
be created on that partition.</p>
<p>Mailboxes previously created on such partitions are of course still
accessible, and subfolders are by default still created on the same
partition as the parent folder.</p>
</div>
</section>
<section id="partition-s-usage-information-reset">
<h3>Partition's Usage Information Reset<a class="headerlink" href="#partition-s-usage-information-reset" title="Permalink to this heading"></a></h3>
<p>By default, partition's usage information is retrieved only once -- when
the service first initializes.</p>
<p>If you tend to use the same service instance for a long lapse of time
and performs a large amount of mailbox creations, it may be useful to
configure the <code class="docutils literal notranslate"><span class="pre">partition_select_usage_reinit</span></code> setting so that
each partition's usage information is refreshed after the configured
number of creation requests.</p>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>This only concerns selection modes other than <cite>random`</cite>.</p>
</div>
</section>
</section>
<section id="backend-selection-mode-cyrus-imap-murder">
<h2>Backend Selection Mode (Cyrus IMAP Murder)<a class="headerlink" href="#backend-selection-mode-cyrus-imap-murder" title="Permalink to this heading"></a></h2>
<p>Upon creating a user mailbox, if the target server is not given as
extra parameter, the mailbox is created on either;</p>
<ul class="simple">
<li><p>the configured <code class="docutils literal notranslate"><span class="pre">defaultserver</span></code> backend</p></li>
<li><p>the most fitting backend (partition) among the servers listed in the
<code class="docutils literal notranslate"><span class="pre">serverlist</span></code> setting, if <code class="docutils literal notranslate"><span class="pre">defaultserver</span></code> is not configured.</p></li>
</ul>
<p class="rubric">Related Settings</p>
<ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre">serverlist_select_mode</span></code></p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">serverlist_select_usage_reinit</span></code></p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">serverlist_select_soft_usage_limit</span></code></p></li>
</ul>
<section id="available-selection-modes-on-frontend">
<h3>Available Selection Modes on Frontend<a class="headerlink" href="#available-selection-modes-on-frontend" title="Permalink to this heading"></a></h3>
<p><strong>random</strong></p>
<blockquote>
<div><p>The selection is (pseudo-)random. Each backend has the same
probability of being selected.</p>
</div></blockquote>
<aside class="sidebar">
<p class="sidebar-title">Example Scenario for Selection Modes</p>
<p>Suppose that the partitions configured are:</p>
<table class="docutils align-default">
<thead>
<tr class="row-odd"><th class="head"><p>Backend</p></th>
<th class="head"><p>Partition</p></th>
<th class="head"><p>Total</p></th>
<th class="head"><p>Free</p></th>
<th class="head"><p>%</p></th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">be1</span></code></p></td>
<td></td>
<td><p>2000 GB</p></td>
<td><p>1000 GB</p></td>
<td><p>50%</p></td>
</tr>
<tr class="row-odd"><td></td>
<td><p><code class="docutils literal notranslate"><span class="pre">part1</span></code></p></td>
<td><p>1000 GB</p></td>
<td><p>500 GB</p></td>
<td><p>50%</p></td>
</tr>
<tr class="row-even"><td></td>
<td><p><code class="docutils literal notranslate"><span class="pre">part2</span></code></p></td>
<td><p>1000 GB</p></td>
<td><p>500 GB</p></td>
<td><p>50%</p></td>
</tr>
<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">be2</span></code></p></td>
<td></td>
<td><p>2000 GB</p></td>
<td><p>900 GB</p></td>
<td><p>45%</p></td>
</tr>
<tr class="row-even"><td></td>
<td><p><code class="docutils literal notranslate"><span class="pre">part1</span></code></p></td>
<td><p>1000 GB</p></td>
<td><p>200 GB</p></td>
<td><p>20%</p></td>
</tr>
<tr class="row-odd"><td></td>
<td><p><code class="docutils literal notranslate"><span class="pre">part2</span></code></p></td>
<td><p>1000 GB</p></td>
<td><p>700 GB</p></td>
<td><p>70%</p></td>
</tr>
<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">be3</span></code></p></td>
<td></td>
<td><p>200 GB</p></td>
<td><p>110 GB</p></td>
<td><p>55%</p></td>
</tr>
<tr class="row-odd"><td></td>
<td><p><code class="docutils literal notranslate"><span class="pre">part1</span></code></p></td>
<td><p>100 GB</p></td>
<td><p>30 GB</p></td>
<td><p>30%</p></td>
</tr>
<tr class="row-even"><td></td>
<td><p><code class="docutils literal notranslate"><span class="pre">part2</span></code></p></td>
<td><p>100 GB</p></td>
<td><p>80 GB</p></td>
<td><p>80%</p></td>
</tr>
</tbody>
</table>
</aside>
<p><strong>freespace-most</strong></p>
<blockquote>
<div><p>The backend which has the most absolute free space (counted in KiB
units) is selected.</p>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>The considered free space is the sum of the free space on all
available partitions on the backend.</p>
</div>
<p>In the example scenario, <code class="docutils literal notranslate"><span class="pre">be1</span></code> would be selected as most fitting,
since 1000 GB of free space is the most of all backends.</p>
</div></blockquote>
<p><strong>freespace-percent-most</strong></p>
<blockquote>
<div><p>On each backend, only the partition with the most percentage of free
space is considered. The selected backend is the one whose partition
has the highest percentage of free space.</p>
<p>In the example scenario, <code class="docutils literal notranslate"><span class="pre">be3</span></code> would be selected as most fitting,
since it has a partition with 80% of free space which is the
highest of all backends.</p>
</div></blockquote>
<p><strong>freespace-percent-weighted</strong></p>
<blockquote>
<div><p>On each backend, only the partition with the most percentage of free
space is considered: this is the backend weight. Then a weighted
choice is performed to select one of the backends.</p>
<p>In the example scenario, the weight of each backend would be:</p>
<ul class="simple">
<li><p>50 for <code class="docutils literal notranslate"><span class="pre">be1</span></code></p></li>
<li><p>70 for <code class="docutils literal notranslate"><span class="pre">be2</span></code></p></li>
<li><p>80 for <code class="docutils literal notranslate"><span class="pre">be3</span></code></p></li>
</ul>
<p>The sum of all weights being 200, the probability for each backend
to be selected as most fitting would be:</p>
<ul class="simple">
<li><p>25% for <code class="docutils literal notranslate"><span class="pre">be1</span></code></p></li>
<li><p>35% for <code class="docutils literal notranslate"><span class="pre">be2</span></code></p></li>
<li><p>40% for <code class="docutils literal notranslate"><span class="pre">be3</span></code></p></li>
</ul>
</div></blockquote>
<p><strong>freespace-percent-weighted-delta</strong></p>
<blockquote>
<div><p>On each backend, only the partition with the most percentage of free
space is considered.</p>
<p>Like with <code class="docutils literal notranslate"><span class="pre">freespace-percent-weighted</span></code>, a weight is associated to
each backend. It is computed as follows:</p>
<div class="math notranslate nohighlight">
\[free - leastfree + 0.5\]</div>
<p>Then a weighted choice is performed to select one of the backends.</p>
<p>In the example scenario, the weight of each backend would be:</p>
<ul class="simple">
<li><p>For <code class="docutils literal notranslate"><span class="pre">be1</span></code>: <span class="math notranslate nohighlight">\(50 - 50 + 0.5 = 0.5\)</span></p></li>
<li><p>For <code class="docutils literal notranslate"><span class="pre">be2</span></code>: <span class="math notranslate nohighlight">\(70 - 50 + 0.5 = 20.5\)</span></p></li>
<li><p>For <code class="docutils literal notranslate"><span class="pre">be3</span></code>: <span class="math notranslate nohighlight">\(80 - 50 + 0.5 = 30.5\)</span></p></li>
</ul>
<p>Then the probability for each backend to be selected as most fitting would be:</p>
<ul class="simple">
<li><p>1.0% for <code class="docutils literal notranslate"><span class="pre">be1</span></code></p></li>
<li><p>39.8% for <code class="docutils literal notranslate"><span class="pre">be2</span></code></p></li>
<li><p>59.2% for <code class="docutils literal notranslate"><span class="pre">be3</span></code></p></li>
</ul>
</div></blockquote>
</section>
<section id="excluding-backends">
<h3>Excluding Backends<a class="headerlink" href="#excluding-backends" title="Permalink to this heading"></a></h3>
<p>When using a selection mode other than <code class="docutils literal notranslate"><span class="pre">random</span></code>, backends are
automatically excluded if their considered usage percentage is beyond
the <code class="docutils literal notranslate"><span class="pre">serverlist_select_soft_usage_limit</span></code> integer setting.</p>
</section>
<section id="backend-s-usage-information-reset">
<h3>Backend's Usage Information Reset<a class="headerlink" href="#backend-s-usage-information-reset" title="Permalink to this heading"></a></h3>
<p>By default backends usage data are retrieved only once upon service
initialization.</p>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>This only concerns selection modes other than <code class="docutils literal notranslate"><span class="pre">random</span></code>.</p>
</div>
<p>If you tend to use the same service instance for a long lapse of time
and performs a large amounts of mailbox creations, it may be useful to
configure the <code class="docutils literal notranslate"><span class="pre">serverlist_select_usage_reinit</span></code> so that the backend's
disk usage information is refreshed after the configured number of
creation requests.</p>
<p>Back to <a class="reference internal" href="../features.html#imap-features"><span class="std std-ref">Features</span></a></p>
</section>
</section>
</section>
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