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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-20150] Chunk migration locks are constantly blocking map/reduce</title>
                <link>https://jira.mongodb.org/browse/SERVER-20150</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;Hi.&lt;/p&gt;

&lt;p&gt;Please see &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-20149&quot; title=&quot;WiredTiger read performance&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-20149&quot;&gt;&lt;del&gt;SERVER-20149&lt;/del&gt;&lt;/a&gt; for our deployment details.&lt;/p&gt;

&lt;p&gt;We&apos;re seeing another problem with shard balancer. After upgrade our app failed to perform map/reduce on sharded collections with following error:&lt;/p&gt;

&lt;p/&gt;
&lt;div id=&quot;syntaxplugin&quot; class=&quot;syntaxplugin&quot; style=&quot;border: 1px dashed #bbb; border-radius: 5px !important; overflow: auto; max-height: 30em;&quot;&gt;
&lt;table cellspacing=&quot;0&quot; cellpadding=&quot;0&quot; border=&quot;0&quot; width=&quot;100%&quot; style=&quot;font-size: 1em; line-height: 1.4em !important; font-weight: normal; font-style: normal; color: black;&quot;&gt;
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					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;  margin-top: 10px;   margin-bottom: 10px;  width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;[conn26] the collection metadata could not be locked for mapreduce, already locked by { _id: &quot;&amp;lt;db&amp;gt;.&amp;lt;collection&amp;gt;&quot;, process: &quot;db03:27017:1440329966:296879767&quot;, state: 2, ts: ObjectId(&apos;55da2f929342a2275e4eb52f&apos;), when: new Date(1440362386369), who: &quot;db03:27017:1440329966:296879767:conn5087:1423199152&quot;, why: &quot;migrating chunk [{ : MinKey }, { : MaxKey }) in db&amp;gt;.&amp;lt;collection&amp;gt;&quot; }&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
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&lt;p/&gt;

&lt;p&gt;I&apos;ve checked the status of the shard, and found that there a chunk migration is pending from Shard3 to Shard1, since there a imbalance in chunks distribution:&lt;/p&gt;

&lt;p/&gt;
&lt;div id=&quot;syntaxplugin&quot; class=&quot;syntaxplugin&quot; style=&quot;border: 1px dashed #bbb; border-radius: 5px !important; overflow: auto; max-height: 30em;&quot;&gt;
&lt;table cellspacing=&quot;0&quot; cellpadding=&quot;0&quot; border=&quot;0&quot; width=&quot;100%&quot; style=&quot;font-size: 1em; line-height: 1.4em !important; font-weight: normal; font-style: normal; color: black;&quot;&gt;
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					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;  margin-top: 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;      chunks:&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
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						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;                                shard1     713&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
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				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;                                shard2     715&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
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				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   margin-bottom: 10px;  width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;                                shard3     812&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
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			&lt;/tbody&gt;
&lt;/table&gt;
&lt;/div&gt;
&lt;p/&gt;

&lt;p&gt;I&apos;ve checked the db.opStatus on &lt;tt&gt;shard1&lt;/tt&gt; primary node and found out that migration process is blocked by secondary node, because it&apos;s doing the initial sync. We&apos;ve decided to stop the initial sync to give primary node time to accept chunks from shard3. But after ~2 hours only 2 chunks are actually migrated and our collection was still locked. So we decided to stop the balancer, to allow our app to run again. &lt;/p&gt;

&lt;p&gt;Is this by design or something really went wrong during upgrade proccess? Because we haven&apos;t seen this issue before on 2.4 installation.&lt;/p&gt;</description>
                <environment></environment>
        <key id="227227">SERVER-20150</key>
            <summary>Chunk migration locks are constantly blocking map/reduce</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="sam.kleinman">Sam Kleinman</assignee>
                                    <reporter username="yopp">Alex</reporter>
                        <labels>
                    </labels>
                <created>Wed, 26 Aug 2015 20:21:55 +0000</created>
                <updated>Fri, 11 Sep 2015 20:08:27 +0000</updated>
                            <resolved>Fri, 11 Sep 2015 19:00:59 +0000</resolved>
                                    <version>3.0.5</version>
                                                    <component>Sharding</component>
                                        <votes>0</votes>
                                    <watches>5</watches>
                                                                                                                <comments>
                            <comment id="1027865" author="yopp" created="Fri, 11 Sep 2015 20:08:27 +0000"  >&lt;p&gt;Hi Sam,&lt;/p&gt;

&lt;p&gt;1) Are you sure this is the expected behaviour for the Shard Balancer to lock a collection for &lt;b&gt;hours&lt;/b&gt; (2 hours in our case)?&lt;br/&gt;
2) We believe that this is a regression: we haven&apos;t had this problem on 2.4.&lt;/p&gt;

&lt;p&gt;Otherwise, this means that regardless of what is written in the docs we simply can&apos;t use a sharded collection as map/reduce output since it will likely be locked by the balancer. &lt;/p&gt;
</comment>
                            <comment id="1027778" author="samk" created="Fri, 11 Sep 2015 19:00:50 +0000"  >&lt;p&gt;Sorry for the delay in getting back to you. &lt;/p&gt;

&lt;p&gt;After discussing this with the teams that work on sharding, it looks like this is in fact the expected behavior: &lt;tt&gt;mapReduce&lt;/tt&gt; require the distributed lock to prevent chunk migrations when running with sharded output to prevent chunk migrations from interfering with the output of the map reduce operation. &lt;/p&gt;

&lt;p&gt;I hope this makes sense, and sorry for any confusion.&lt;/p&gt;

&lt;p&gt;Regards,&lt;br/&gt;
sam&lt;/p&gt;</comment>
                            <comment id="1015395" author="yopp" created="Wed, 26 Aug 2015 20:25:37 +0000"  >&lt;p&gt;Oopsie, task should be &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-20149&quot; title=&quot;WiredTiger read performance&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-20149&quot;&gt;&lt;del&gt;SERVER-20149&lt;/del&gt;&lt;/a&gt;&lt;/p&gt;</comment>
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