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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>[DOCS-2211] Documentation on Secondary Throttle does not describe what it actually does</title>
                <link>https://jira.mongodb.org/browse/DOCS-2211</link>
                <project id="10380" key="DOCS">Documentation</project>
                    <description>&lt;p&gt;The documentation on Secondary Throttle does not accurately describe how it changes the behavior of migration.&lt;/p&gt;

&lt;p&gt;When Secondary Throttle is &lt;ins&gt;not&lt;/ins&gt; set, then the migration code will copy the documents from the FROM shard to the TO shard as quickly as it can, without waiting for replication.  When all of the documents have been copied, but before the migration enters the critical section, the FROM shard will wait for replication to complete to a &lt;b&gt;&lt;em&gt;MAJORITY&lt;/em&gt;&lt;/b&gt; of the nodes in the destination shard.&lt;/p&gt;

&lt;p&gt;Ref: &lt;a href=&quot;https://github.com/mongodb/mongo/blob/r2.4.7/src/mongo/s/d_migrate.cpp#L1965-L1980&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/blob/r2.4.7/src/mongo/s/d_migrate.cpp#L1965-L1980&lt;/a&gt;&lt;/p&gt;

&lt;p&gt;This has the effect of creating a bottleneck right at the end of the document movement, as the migration needs to wait for all of the writes to replicate.&lt;/p&gt;

&lt;p&gt;When Secondary Throttle &lt;ins&gt;is&lt;/ins&gt; set, then the FROM shard will wait for replication to complete to a majority of the nodes in the TO shard after &lt;ins&gt;each&lt;/ins&gt; document is inserted in the TO shard.  &lt;/p&gt;

&lt;p&gt;Ref: &lt;a href=&quot;https://github.com/mongodb/mongo/blob/r2.4.7/src/mongo/s/d_migrate.cpp#L905-L911&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/blob/r2.4.7/src/mongo/s/d_migrate.cpp#L905-L911&lt;/a&gt;&lt;/p&gt;

&lt;p&gt;In addition to this, the FROM shard will always wait for the replication to complete to the TO shard before entering the critical section.&lt;/p&gt;

&lt;p&gt;The net effect of this is to smooth out the load and reduce the overall load on the replica set.&lt;/p&gt;

&lt;p&gt;The current documentation is incorrect in two ways:&lt;/p&gt;
&lt;ul class=&quot;alternate&quot; type=&quot;square&quot;&gt;
	&lt;li&gt;It implies that there is &lt;b&gt;no&lt;/b&gt; wait for replication if Secondary Throttle is not set&lt;/li&gt;
	&lt;li&gt;It gives the write concern for Secondary Throttle as {w:2}, when it should really be {w:&quot;majority&quot;}&lt;/li&gt;
&lt;/ul&gt;
</description>
                <environment>&lt;a href=&quot;http://docs.mongodb.org/manual/tutorial/configure-sharded-cluster-balancer/#require-replication-before-chunk-migration-secondary-throttle&quot;&gt;http://docs.mongodb.org/manual/tutorial/configure-sharded-cluster-balancer/#require-replication-before-chunk-migration-secondary-throttle&lt;/a&gt;</environment>
        <key id="97704">DOCS-2211</key>
            <summary>Documentation on Secondary Throttle does not describe what it actually does</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="9">Done</resolution>
                                        <assignee username="kay.kim@mongodb.com">Kay Kim</assignee>
                                    <reporter username="william.zola@10gen.com">William Zola</reporter>
                        <labels>
                    </labels>
                <created>Fri, 8 Nov 2013 01:29:28 +0000</created>
                <updated>Wed, 11 Jan 2017 22:38:50 +0000</updated>
                            <resolved>Thu, 16 Jan 2014 15:21:15 +0000</resolved>
                                                    <fixVersion>01112017-cleanup</fixVersion>
                                        <due></due>
                            <votes>0</votes>
                                    <watches>4</watches>
                                                                                                                <comments>
                            <comment id="483076" author="xgen-internal-githook" created="Thu, 16 Jan 2014 15:18:22 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{u&apos;username&apos;: u&apos;kay-kim&apos;, u&apos;name&apos;: u&apos;kay&apos;, u&apos;email&apos;: u&apos;kay.kim@10gen.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/DOCS-2079&quot; title=&quot;Clarify when the _secondaryThrottle setting would take effect after changing it&quot; class=&quot;issue-link&quot; data-issue-key=&quot;DOCS-2079&quot;&gt;&lt;del&gt;DOCS-2079&lt;/del&gt;&lt;/a&gt; &lt;a href=&quot;https://jira.mongodb.org/browse/DOCS-2211&quot; title=&quot;Documentation on Secondary Throttle does not describe what it actually does&quot; class=&quot;issue-link&quot; data-issue-key=&quot;DOCS-2211&quot;&gt;&lt;del&gt;DOCS-2211&lt;/del&gt;&lt;/a&gt; chunk migration and replication&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/docs/commit/40cdb204d66ba9e3c6e7f8930c731f1d504f39a2&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/docs/commit/40cdb204d66ba9e3c6e7f8930c731f1d504f39a2&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="483071" author="xgen-internal-githook" created="Thu, 16 Jan 2014 15:18:12 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{u&apos;username&apos;: u&apos;Zackrobat&apos;, u&apos;name&apos;: u&apos;Zack Brown&apos;, u&apos;email&apos;: u&apos;zack.brown@10gen.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/DOCS-2079&quot; title=&quot;Clarify when the _secondaryThrottle setting would take effect after changing it&quot; class=&quot;issue-link&quot; data-issue-key=&quot;DOCS-2079&quot;&gt;&lt;del&gt;DOCS-2079&lt;/del&gt;&lt;/a&gt; and &lt;a href=&quot;https://jira.mongodb.org/browse/DOCS-2211&quot; title=&quot;Documentation on Secondary Throttle does not describe what it actually does&quot; class=&quot;issue-link&quot; data-issue-key=&quot;DOCS-2211&quot;&gt;&lt;del&gt;DOCS-2211&lt;/del&gt;&lt;/a&gt;: correct secondary throttle explanation&lt;/p&gt;

&lt;p&gt;Signed-off-by: kay &amp;lt;kay.kim@10gen.com&amp;gt;&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/docs/commit/efcfe079f9de78f937cd755dc79be1b362bdbf9a&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/docs/commit/efcfe079f9de78f937cd755dc79be1b362bdbf9a&lt;/a&gt;&lt;/p&gt;</comment>
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