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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-68670] Cache imbalanced collections for balancing</title>
                <link>https://jira.mongodb.org/browse/SERVER-68670</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;At the beginning of every balancing round, data size stats are collected from each shards in order to spot potential imbalance and schedule migrations. Currently, the balancer is requesting such stats in batches, collecting info for up to &lt;a href=&quot;https://github.com/mongodb/mongo/blob/9f8caddfcefb517993b531d74689b2d98192ef96/src/mongo/db/s/balancer/balancer_chunk_selection_policy_impl.cpp#L428&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;20 collections per request&lt;/a&gt;.&lt;/p&gt;

&lt;p&gt;Purpose of this ticket is to rethink the current batching policy because this hardcoded limit may become source of scalability issues in case of thousands of sharded collections.&lt;/p&gt;

&lt;p&gt;&lt;font color=&quot;#DE350B&quot;&gt;&lt;span class=&quot;error&quot;&gt;&amp;#91;EDIT&amp;#93;&lt;/span&gt; REPURPOSED&lt;/font&gt;: this ticket also added a cache of imbalanced collections that lives between rounds in order to remember which collections had a migration scheduled during the previous round.&lt;/p&gt;</description>
                <environment></environment>
        <key id="2109463">SERVER-68670</key>
            <summary>Cache imbalanced collections for balancing</summary>
                <type id="1" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14703&amp;avatarType=issuetype">Bug</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="silvia.surroca@mongodb.com">Silvia Surroca</assignee>
                                    <reporter username="pierlauro.sciarelli@mongodb.com">Pierlauro Sciarelli</reporter>
                        <labels>
                            <label>balancer-perf-improvement</label>
                    </labels>
                <created>Tue, 9 Aug 2022 12:50:05 +0000</created>
                <updated>Sun, 29 Oct 2023 21:34:45 +0000</updated>
                            <resolved>Thu, 23 Mar 2023 13:28:59 +0000</resolved>
                                                    <fixVersion>7.0.0-rc0</fixVersion>
                    <fixVersion>6.0.6</fixVersion>
                    <fixVersion>6.3.2</fixVersion>
                                                        <votes>0</votes>
                                    <watches>6</watches>
                                                                                                                <comments>
                            <comment id="5370736" author="xgen-internal-githook" created="Mon, 24 Apr 2023 15:15:35 +0000"  >&lt;p&gt;Author: &lt;/p&gt;
{&apos;name&apos;: &apos;Pol Pinol Castuera&apos;, &apos;email&apos;: &apos;pol.pinol@mongodb.com&apos;, &apos;username&apos;: &apos;PolPinol&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-68670&quot; title=&quot;Cache imbalanced collections for balancing&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-68670&quot;&gt;&lt;del&gt;SERVER-68670&lt;/del&gt;&lt;/a&gt; Collecting balancing stats must not impact scalability&lt;br/&gt;
Branch: v6.0&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/2bd08faf234eab61f838532998d021dafb50951e&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/2bd08faf234eab61f838532998d021dafb50951e&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="5362310" author="xgen-internal-githook" created="Thu, 20 Apr 2023 13:36:25 +0000"  >&lt;p&gt;Author: &lt;/p&gt;
{&apos;name&apos;: &apos;Silvia Surroca&apos;, &apos;email&apos;: &apos;silvia.surroca@mongodb.com&apos;, &apos;username&apos;: &apos;silviasuhu&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-68670&quot; title=&quot;Cache imbalanced collections for balancing&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-68670&quot;&gt;&lt;del&gt;SERVER-68670&lt;/del&gt;&lt;/a&gt; Collecting balancing stats must not impact scalability&lt;br/&gt;
Branch: v6.3&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/69c396199a7bcf409c02e456d192fa794f03aef7&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/69c396199a7bcf409c02e456d192fa794f03aef7&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="5295052" author="xgen-internal-githook" created="Thu, 23 Mar 2023 12:10:32 +0000"  >&lt;p&gt;Author: &lt;/p&gt;
{&apos;name&apos;: &apos;Silvia Surroca&apos;, &apos;email&apos;: &apos;silvia.surroca@mongodb.com&apos;, &apos;username&apos;: &apos;silviasuhu&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-68670&quot; title=&quot;Cache imbalanced collections for balancing&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-68670&quot;&gt;&lt;del&gt;SERVER-68670&lt;/del&gt;&lt;/a&gt; Collecting balancing stats must not impact scalability&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/3db39116e26f32e3a53ad55140783f4e0b01147f&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/3db39116e26f32e3a53ad55140783f4e0b01147f&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="5291347" author="xgen-internal-githook" created="Wed, 22 Mar 2023 10:31:24 +0000"  >&lt;p&gt;Author: &lt;/p&gt;
{&apos;name&apos;: &apos;Silvia Surroca&apos;, &apos;email&apos;: &apos;silvia.surroca@mongodb.com&apos;, &apos;username&apos;: &apos;silviasuhu&apos;}
&lt;p&gt;Message: Revert &quot;&lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-68670&quot; title=&quot;Cache imbalanced collections for balancing&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-68670&quot;&gt;&lt;del&gt;SERVER-68670&lt;/del&gt;&lt;/a&gt; Collecting balancing stats must not impact scalability&quot;&lt;/p&gt;

&lt;p&gt;This reverts commit 9fe03fd6c85760920398b7891fde74069f5457db.&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/b4aec5316dc08fbb8c1d0b48c1477a5a8f086847&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/b4aec5316dc08fbb8c1d0b48c1477a5a8f086847&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="5287306" author="xgen-internal-githook" created="Tue, 21 Mar 2023 08:58:13 +0000"  >&lt;p&gt;Author: &lt;/p&gt;
{&apos;name&apos;: &apos;Silvia Surroca&apos;, &apos;email&apos;: &apos;silvia.surroca@mongodb.com&apos;, &apos;username&apos;: &apos;silviasuhu&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-68670&quot; title=&quot;Cache imbalanced collections for balancing&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-68670&quot;&gt;&lt;del&gt;SERVER-68670&lt;/del&gt;&lt;/a&gt; Collecting balancing stats must not impact scalability&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/9fe03fd6c85760920398b7891fde74069f5457db&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/9fe03fd6c85760920398b7891fde74069f5457db&lt;/a&gt;&lt;/p&gt;</comment>
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