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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-84187] Regression in balancer round storage statistics retrieval</title>
                <link>https://jira.mongodb.org/browse/SERVER-84187</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;In every balancing round, &lt;a href=&quot;https://github.com/mongodb/mongo/blob/9a28587ab0b902ad6e3aa71bb525c61bdd0f69d9/src/mongo/db/s/balancer/cluster_statistics_impl.cpp#L151-L158&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;we retrieve per-shard storage statics&lt;/a&gt;. Since this is done serially, in a cluster with several shards this can be particularly slow, limiting considerably the balancing speed.&lt;/p&gt;

&lt;p&gt;Back in &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-30060&quot; title=&quot;Make the balancer gather storage statistics only for shards which have `maxSize` set&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-30060&quot;&gt;&lt;del&gt;SERVER-30060&lt;/del&gt;&lt;/a&gt; we did an &lt;a href=&quot;https://github.com/mongodb/mongo/commit/e0136739285c097a7da59ba54d6bcd109bb184b5#diff-0b472faf08fa9eb2f954c2acc0139a9f58139beb4af3bebce5ea122def5ec72cL120-R127&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;optimization to retrieve storage statistics only from shards that have maxSize configured&lt;/a&gt;.&lt;/p&gt;

&lt;p&gt;Recently as part of &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-66297&quot; title=&quot;Get rid of `maxSize` for shards&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-66297&quot;&gt;&lt;del&gt;SERVER-66297&lt;/del&gt;&lt;/a&gt; we removed this optimization, so now we retrieve again statistics from all the shards serially at every round.&lt;/p&gt;

&lt;p&gt;The implementation of these statistics retrieval is affected by multiple performance issues:&lt;/p&gt;
&lt;ul&gt;
	&lt;li&gt;Balancer retrieve storage statistics &lt;a href=&quot;https://github.com/mongodb/mongo/blob/9a28587ab0b902ad6e3aa71bb525c61bdd0f69d9/src/mongo/db/s/balancer/cluster_statistics_impl.cpp#L151-L158&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;serially from all shards.&lt;/a&gt;&lt;br/&gt;
&lt;b&gt;Proposal:&lt;/b&gt; Parallelize the retrieval.&lt;/li&gt;
&lt;/ul&gt;


&lt;p&gt;&#160;&lt;/p&gt;
&lt;ul&gt;
	&lt;li&gt;Balancer retrieve storage statistics multiple times in a single balancer round
	&lt;ul&gt;
		&lt;li&gt;&lt;a href=&quot;https://github.com/mongodb/mongo/blob/d7a2e60641011f3cb67bada02fed1b8954bc1e49/src/mongo/db/s/balancer/balancer_chunk_selection_policy.cpp#L328&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;selectChunksToSplit&lt;/a&gt;&lt;/li&gt;
		&lt;li&gt;&lt;a href=&quot;https://github.com/mongodb/mongo/blob/d7a2e60641011f3cb67bada02fed1b8954bc1e49/src/mongo/db/s/balancer/balancer.cpp#L819&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;selectChunksToMove&lt;/a&gt;&lt;/li&gt;
		&lt;li&gt;&lt;a href=&quot;https://github.com/mongodb/mongo/blob/d7a2e60641011f3cb67bada02fed1b8954bc1e49/src/mongo/db/s/balancer/balancer.cpp#L791&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;warnOnMultiversion&lt;/a&gt;&lt;/li&gt;
	&lt;/ul&gt;
	&lt;/li&gt;
&lt;/ul&gt;


&lt;p&gt;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; &lt;b&gt;Proposal:&lt;/b&gt; Do it only once per round and re-use the same &lt;tt&gt;ClusterStatistcs&lt;/tt&gt; object&lt;/p&gt;</description>
                <environment></environment>
        <key id="2525119">SERVER-84187</key>
            <summary>Regression in balancer round storage statistics retrieval</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="1" iconUrl="https://jira.mongodb.org/images/icons/statuses/open.png" description="">Open</status>
                    <statusCategory id="2" key="new" colorName="default"/>
                                    <resolution id="-1">Unresolved</resolution>
                                        <assignee username="backlog-server-catalog-and-routing">Backlog - Catalog and Routing</assignee>
                                    <reporter username="tommaso.tocci@mongodb.com">Tommaso Tocci</reporter>
                        <labels>
                            <label>balancer-round-perf</label>
                            <label>car-qw</label>
                    </labels>
                <created>Thu, 14 Dec 2023 17:59:55 +0000</created>
                <updated>Thu, 21 Dec 2023 11:28:08 +0000</updated>
                                            <version>7.0.0</version>
                    <version>7.2.0-rc0</version>
                    <version>7.1.0</version>
                    <version>7.3.0-rc0</version>
                                                                        <votes>0</votes>
                                    <watches>1</watches>
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            <issuekey id="2041524">SERVER-66297</issuekey>
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