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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-61557] Get rid of top chunk migration</title>
                <link>https://jira.mongodb.org/browse/SERVER-61557</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;The top chunk migration &lt;a href=&quot;https://github.com/mongodb/mongo/blob/25993169a43f40d4f13d2eba8e6645598b62cc1c/src/mongo/db/s/chunk_splitter.cpp#L366-L368&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;was thought&lt;/a&gt; as a way to distribute the write load for applications inserting a lot of documents with non-hashed incremental shard keys.&lt;/p&gt;

&lt;p&gt;It turns out that for heavy workloads it could cause more harm than good:&lt;/p&gt;
&lt;ul class=&quot;alternate&quot; type=&quot;square&quot;&gt;
	&lt;li&gt;Trying to move the right-most chunk while writing on it could result in very long migrations that can&apos;t reach the catch-up phase and eventually fail after hours.&lt;/li&gt;
	&lt;li&gt;In a balanced environment, it results in ping-pong (moving continuously the top chunk on another shard, then split, then move back) that can bring to increased migrations unavailability due to the waiting for overlapping range deletions to finish on the receiver.&lt;/li&gt;
&lt;/ul&gt;
</description>
                <environment></environment>
        <key id="1927387">SERVER-61557</key>
            <summary>Get rid of top chunk migration</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="3">Duplicate</resolution>
                                        <assignee username="pierlauro.sciarelli@mongodb.com">Pierlauro Sciarelli</assignee>
                                    <reporter username="pierlauro.sciarelli@mongodb.com">Pierlauro Sciarelli</reporter>
                        <labels>
                            <label>shardingemea-qw</label>
                    </labels>
                <created>Wed, 17 Nov 2021 13:59:41 +0000</created>
                <updated>Tue, 16 May 2023 08:41:54 +0000</updated>
                            <resolved>Mon, 20 Mar 2023 13:36:33 +0000</resolved>
                                                                    <component>Sharding</component>
                                        <votes>0</votes>
                                    <watches>9</watches>
                                                                                                                <comments>
                            <comment id="5427379" author="xgen-internal-githook" created="Tue, 16 May 2023 08:41:54 +0000"  >&lt;p&gt;Author: &lt;/p&gt;
{&apos;name&apos;: &apos;Pierlauro Sciarelli&apos;, &apos;email&apos;: &apos;pierlauro.sciarelli@mongodb.com&apos;, &apos;username&apos;: &apos;pierlauro&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-75065&quot; title=&quot;Complete TODO listed in SERVER-61557&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-75065&quot;&gt;&lt;del&gt;SERVER-75065&lt;/del&gt;&lt;/a&gt; Complete TODO listed in &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-61557&quot; title=&quot;Get rid of top chunk migration&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-61557&quot;&gt;&lt;del&gt;SERVER-61557&lt;/del&gt;&lt;/a&gt;&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/31572586adb8aac04d930ca56887ca742bd2c048&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/31572586adb8aac04d930ca56887ca742bd2c048&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="5284609" author="pierlauro.sciarelli" created="Mon, 20 Mar 2023 13:36:33 +0000"  >&lt;p&gt;Closing as duplicate of &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-66652&quot; title=&quot;Remove NoMoreAutoSplitter feature flag&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-66652&quot;&gt;&lt;del&gt;SERVER-66652&lt;/del&gt;&lt;/a&gt; that got rid of the chunk splitter that was the only component issuing top chunk migrations&lt;/p&gt;</comment>
                            <comment id="4582826" author="pierlauro.sciarelli" created="Tue, 31 May 2022 13:28:48 +0000"  >&lt;p&gt;I would keep separate tickets.&lt;/p&gt;

&lt;p&gt;We protect only the triggering of the top chunk migration flow via the feature flag, but not all code used for performing top chunk optimization is protected by the feature flag. For example, &lt;a href=&quot;https://github.com/mongodb/mongo/blob/84b97380717f3e04f7aea56cab0074dc93f58663/src/mongo/db/s/balancer/balancer.cpp#L327&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;Balancer::rebalanceSingleChunk&lt;/a&gt; is exclusively used by top chunk optimization, but there is no reference to the feature flag.&lt;/p&gt;</comment>
                            <comment id="4582762" author="tommaso.tocci" created="Tue, 31 May 2022 13:12:17 +0000"  >&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=pierlauro.sciarelli%40mongodb.com&quot; class=&quot;user-hover&quot; rel=&quot;pierlauro.sciarelli@mongodb.com&quot;&gt;pierlauro.sciarelli@mongodb.com&lt;/a&gt; could you confirm that the entire code of the top chunk optimization will go away with the autosplitter? If so can we mark this as duplicate of the NOMAS feature flag removal&lt;/p&gt;</comment>
                            <comment id="4197290" author="kaloian.manassiev" created="Thu, 18 Nov 2021 12:38:34 +0000"  >&lt;p&gt;I would like to turn this into a project around improving the behaviour of bulk-loading of data into a new sharded cluster.&lt;/p&gt;</comment>
                    </comments>
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            <issuekey id="2019225">SERVER-65322</issuekey>
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            <issuekey id="978447">SERVER-44192</issuekey>
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