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    <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-2118] Indexing during phase 2 of a Map Reduce execution blocks all db operations</title>
                <link>https://jira.mongodb.org/browse/SERVER-2118</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;When we are running large Map Reduce jobs, we have noticed that an index building operation seems to happen as phase 2 of 3 phases. When this happens, it seems to prevent the database from being used. That is, the indexing is not happening in the background and also not during insertions into the temporary MR collections. &lt;/p&gt;

&lt;p&gt;See output snippet from the currentOp command. This index building seemed to happen during the execution of a Map/Reduce job after the Map phase ended and before the Reduce phase began. &lt;/p&gt;

&lt;p&gt;{&lt;br/&gt;
	&quot;opid&quot; : 853796039,&lt;br/&gt;
	&quot;active&quot; : true,&lt;br/&gt;
	&quot;lockType&quot; : &quot;write&quot;,&lt;br/&gt;
	&quot;waitingForLock&quot; : false,&lt;br/&gt;
	&quot;secs_running&quot; : 229,&lt;br/&gt;
	&quot;op&quot; : &quot;insert&quot;,&lt;br/&gt;
	&quot;ns&quot; : &quot;sociocast.system.indexes&quot;,&lt;br/&gt;
	&quot;client&quot; : &quot;0.0.0.0:0&quot;,&lt;br/&gt;
	&quot;desc&quot; : &quot;conn&quot;,&lt;br/&gt;
	&quot;msg&quot; : &quot;index: (2/3) btree bottom up 4200477/18418261 22%&quot;&lt;br/&gt;
},&lt;/p&gt;</description>
                <environment>Centos 64 bit, Amazon EC2</environment>
        <key id="13733">SERVER-2118</key>
            <summary>Indexing during phase 2 of a Map Reduce execution blocks all db operations</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="5">Cannot Reproduce</resolution>
                                        <assignee username="antoine">Antoine Girbal</assignee>
                                    <reporter username="giri.iyengar">Giri Iyengar</reporter>
                        <labels>
                    </labels>
                <created>Thu, 18 Nov 2010 16:01:46 +0000</created>
                <updated>Wed, 15 Aug 2012 14:02:57 +0000</updated>
                            <resolved>Tue, 29 May 2012 06:44:41 +0000</resolved>
                                    <version>1.6.2</version>
                                                                        <votes>0</votes>
                                    <watches>0</watches>
                                                                                                                <comments>
                            <comment id="123748" author="antoine" created="Tue, 29 May 2012 06:44:41 +0000"  >&lt;p&gt;please reopen if needed&lt;/p&gt;</comment>
                            <comment id="123747" author="antoine" created="Tue, 29 May 2012 06:43:32 +0000"  >&lt;p&gt;this ticket is old..&lt;br/&gt;
Looking at code the only indices created by MR are:&lt;/p&gt;
&lt;ul class=&quot;alternate&quot; type=&quot;square&quot;&gt;
	&lt;li&gt;on &quot;0&quot; for the inc collection&lt;/li&gt;
	&lt;li&gt;on &quot;_id&quot; and potentially other ones for the temp collection, copied over from the pre-existing target collection&lt;br/&gt;
In any case those are all done when the collections are empty, so there is no risk of blocking the database.&lt;/li&gt;
&lt;/ul&gt;
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