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    <title>MongoDB Jira</title>
    <link>https://jira.mongodb.org</link>
    <description>This file is an XML representation of an issue</description>
    <language>en-us</language>    <build-info>
        <version>9.7.1</version>
        <build-number>970001</build-number>
        <build-date>13-04-2023</build-date>
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<item>
            <title>[SERVER-58472] Implement a balancer merge chunks machinery</title>
                <link>https://jira.mongodb.org/browse/SERVER-58472</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;The balancer needs to support new functionality which will be required for the algorithm in SERVER-58471&lt;/p&gt;

&lt;p&gt;After putting the balancer into a &quot;merge chunks&quot; mode via the per-collection command developed as part of  &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-58270&quot; title=&quot;Create configureCollectionAutoSplitter command &quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-58270&quot;&gt;&lt;del&gt;SERVER-58270&lt;/del&gt;&lt;/a&gt; ,&lt;br/&gt;
the balancer needs to work in a special mode where we attempt to merge and move chunks to shards where they can be merged. &lt;/p&gt;

&lt;p&gt;The end goal is to reach a optimal number of chunks based on the default chunk size and the size of the data.&lt;br/&gt;
After the merge chunks algorithm signals convergence, the balancer needs to move back into the default mode.&lt;/p&gt;

&lt;p&gt;&#160;&lt;/p&gt;
&lt;ul&gt;
	&lt;li&gt;Estimate chunk sizes by using the dataSize estimation API&lt;/li&gt;
	&lt;li&gt;Waiting for chunk size estimation should not completely block balancer, if sizes for chunks are unknown we need to queue the estimation operation and then move on to the next operation&lt;/li&gt;
	&lt;li&gt;Store chunk sizes in config.chunk documents for later processing stages&lt;/li&gt;
	&lt;li&gt;Balancer needs to perform merges operations which the merge algorithm decided on&lt;/li&gt;
&lt;/ul&gt;


&lt;p&gt;&lt;a href=&quot;https://github.com/kaloianm/workscripts/blob/master/ctools/defragment_sharded_collection.py&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;See Kal&#8217;s script for proof of concept &lt;/a&gt;&lt;/p&gt;

&lt;p&gt;Below a schematic of the new balancer round&lt;/p&gt;

&lt;p&gt;&lt;span class=&quot;image-wrap&quot; style=&quot;&quot;&gt;&lt;a id=&quot;324831_thumb&quot; href=&quot;https://jira.mongodb.org/secure/attachment/324831/324831_balancer_round.png&quot; title=&quot;balancer_round.png&quot; file-preview-type=&quot;image&quot; file-preview-id=&quot;324831&quot; file-preview-title=&quot;balancer_round.png&quot;&gt;&lt;img src=&quot;https://jira.mongodb.org/secure/thumbnail/324831/_thumb_324831.png&quot; style=&quot;border: 0px solid black&quot; role=&quot;presentation&quot;/&gt;&lt;/a&gt;&lt;/span&gt;&lt;/p&gt;</description>
                <environment></environment>
        <key id="1816255">SERVER-58472</key>
            <summary>Implement a balancer merge chunks machinery</summary>
                <type id="3" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14718&amp;avatarType=issuetype">Task</type>
                                            <priority id="3" iconUrl="https://jira.mongodb.org/images/icons/priorities/major.svg">Major - P3</priority>
                        <status id="6" iconUrl="https://jira.mongodb.org/images/icons/statuses/closed.png" description="The issue is considered finished, the resolution is correct. Issues which are closed can be reopened.">Closed</status>
                    <statusCategory id="3" key="done" colorName="success"/>
                                    <resolution id="13201">Fixed</resolution>
                                        <assignee username="allison.easton@mongodb.com">Allison Easton</assignee>
                                    <reporter username="simon.gratzer@mongodb.com">Simon Gratzer</reporter>
                        <labels>
                    </labels>
                <created>Tue, 13 Jul 2021 11:55:12 +0000</created>
                <updated>Sun, 29 Oct 2023 21:51:01 +0000</updated>
                            <resolved>Wed, 8 Sep 2021 10:53:59 +0000</resolved>
                                    <version>5.1.0</version>
                                    <fixVersion>5.1.0-rc0</fixVersion>
                                                        <votes>0</votes>
                                    <watches>2</watches>
                                                                                                                <comments>
                            <comment id="4107325" author="JIRAUSER1259052" created="Wed, 6 Oct 2021 18:23:29 +0000"  >&lt;p&gt;Updating the fixversion since branching activities occurred yesterday. This ticket will be in rc0 when it&#8217;s been triggered. For more active release information, please keep an eye on #server-release. Thank you!&lt;/p&gt;</comment>
                            <comment id="4036718" author="xgen-internal-githook" created="Thu, 2 Sep 2021 15:42:58 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;Paolo Polato&apos;, &apos;email&apos;: &apos;paolo.polato@mongodb.com&apos;, &apos;username&apos;: &apos;ppolato&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-58472&quot; title=&quot;Implement a balancer merge chunks machinery&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-58472&quot;&gt;&lt;del&gt;SERVER-58472&lt;/del&gt;&lt;/a&gt; First Version of Balancer Commands Scheduler&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/53c498736bef89572d34a5201c3af749f65e68a2&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/53c498736bef89572d34a5201c3af749f65e68a2&lt;/a&gt;&lt;/p&gt;</comment>
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                            <attachment id="324831" name="balancer_round.png" size="95103" author="simon.gratzer@mongodb.com" created="Tue, 13 Jul 2021 11:55:41 +0000"/>
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