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    <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-58211] Deduplicate entries returned in a single invocation to `MigrationChunkClonerSourceLegacy::nextModsBatch`</title>
                <link>https://jira.mongodb.org/browse/SERVER-58211</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;The &lt;a href=&quot;https://github.com/mongodb/mongo/blob/f4276192c880293f536206b19f37c2612208b476/src/mongo/db/s/migration_chunk_cloner_source_legacy.cpp#L721&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;&lt;tt&gt;MigrationChunkClonerSourceLegacy::nextModsBatch&lt;/tt&gt;&lt;/a&gt; call is at the core of transferring the diffs during chunk migration catch-up. As implemented currently:&lt;/p&gt;
&lt;ol&gt;
	&lt;li&gt;During writes we &lt;a href=&quot;https://github.com/mongodb/mongo/blob/4aa92d432b4fe186e051715ff6e910eb92cc544a/src/mongo/db/s/migration_chunk_cloner_source_legacy.cpp#L530&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;place&lt;/a&gt; the {{_id}}s of written documents in a linked list&lt;/li&gt;
	&lt;li&gt;During transferring of the diffs we &lt;a href=&quot;https://github.com/mongodb/mongo/blob/f4276192c880293f536206b19f37c2612208b476/src/mongo/db/s/migration_chunk_cloner_source_legacy.cpp#L993&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;go through that list&lt;/a&gt;, &lt;a href=&quot;https://github.com/mongodb/mongo/blob/f4276192c880293f536206b19f37c2612208b476/src/mongo/db/s/migration_chunk_cloner_source_legacy.cpp#L997&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;fetch&lt;/a&gt; the entire documents and append them to the out buffer&lt;/li&gt;
&lt;/ol&gt;


&lt;p&gt;Because of the above, if the same document was written 2 or more times, it will be present multiple times in the linked list and as a result we will transfer it multiple times. In the case of small updates ($inc, etc) to the same document, this could result in wasteful transfer of documents.&lt;/p&gt;

&lt;p&gt;In order to optimise this, within a single nextModsBatch invocation we should at least keep track of the {{_id}}s that we add to the buffer and make sure we don&apos;t add one twice. In order to make this deduplication as efficient as possible, we should not do BSONObj comparisons, but treat the _id as a bag of bytes.&lt;/p&gt;</description>
                <environment></environment>
        <key id="1807498">SERVER-58211</key>
            <summary>Deduplicate entries returned in a single invocation to `MigrationChunkClonerSourceLegacy::nextModsBatch`</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="kaloian.manassiev@mongodb.com">Kaloian Manassiev</reporter>
                        <labels>
                    </labels>
                <created>Fri, 2 Jul 2021 12:49:21 +0000</created>
                <updated>Sun, 29 Oct 2023 21:51:24 +0000</updated>
                            <resolved>Mon, 26 Jul 2021 13:14:09 +0000</resolved>
                                                    <fixVersion>5.1.0-rc0</fixVersion>
                                    <component>Sharding</component>
                                        <votes>0</votes>
                                    <watches>1</watches>
                                                                                                                <comments>
                            <comment id="4107701" author="JIRAUSER1259052" created="Wed, 6 Oct 2021 18:33:50 +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="3962508" author="xgen-internal-githook" created="Mon, 26 Jul 2021 13:12:18 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;Allison Easton&apos;, &apos;email&apos;: &apos;allison.easton@mongodb.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-58211&quot; title=&quot;Deduplicate entries returned in a single invocation to `MigrationChunkClonerSourceLegacy::nextModsBatch`&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-58211&quot;&gt;&lt;del&gt;SERVER-58211&lt;/del&gt;&lt;/a&gt; Deduplicate entries returned in a single invocation to `MigrationChunkClonerSourceLegacy::nextModsBatch`&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/03de046174e7f3ced4fc099ccc4e1a568c414654&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/03de046174e7f3ced4fc099ccc4e1a568c414654&lt;/a&gt;&lt;/p&gt;</comment>
                    </comments>
                <issuelinks>
                            <issuelinktype id="10020">
                    <name>Gantt Dependency</name>
                                            <outwardlinks description="has to be done before">
                                        <issuelink>
            <issuekey id="1807652">SERVER-58241</issuekey>
        </issuelink>
                            </outwardlinks>
                                                                <inwardlinks description="has to be done after">
                                        <issuelink>
            <issuekey id="1807660">SERVER-58242</issuekey>
        </issuelink>
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