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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>
    </build-info>


<item>
            <title>[SERVER-30660] Replica set fsync blocks secondary reads significantly</title>
                <link>https://jira.mongodb.org/browse/SERVER-30660</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;I have next mongo configuration:&lt;/p&gt;

&lt;p&gt;1. 2 data nodes in replica set&lt;br/&gt;
2. 1 arbiter for the replica set&lt;br/&gt;
3. Java based client&lt;/p&gt;

&lt;p&gt;Both data nodes are configured next way:&lt;br/&gt;
replication:&lt;br/&gt;
 oplogSizeMB: 1024&lt;br/&gt;
 replSetName: arb&lt;/p&gt;

&lt;p&gt;storage:&lt;br/&gt;
 dbPath: /mnt/raid10/mongo&lt;br/&gt;
 journal:&lt;br/&gt;
  enabled: true&lt;br/&gt;
  commitIntervalMs: 500&lt;br/&gt;
 directoryPerDB: true&lt;br/&gt;
 syncPeriodSecs: 60&lt;br/&gt;
 engine: wiredTiger&lt;br/&gt;
 wiredTiger:&lt;br/&gt;
  engineConfig:&lt;br/&gt;
   directoryForIndexes: true&lt;/p&gt;
</description>
                <environment>Linux version 2.6.32-431.el6.x86_64 (gcc version 4.4.7 20120313 (Red Hat 4.4.7-4) (GCC) )</environment>
        <key id="416746">SERVER-30660</key>
            <summary>Replica set fsync blocks secondary reads significantly</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="kelsey.schubert@mongodb.com">Kelsey Schubert</assignee>
                                    <reporter username="dorlov">Denis Orlov</reporter>
                        <labels>
                    </labels>
                <created>Tue, 15 Aug 2017 15:16:07 +0000</created>
                <updated>Mon, 9 Oct 2017 17:01:53 +0000</updated>
                            <resolved>Fri, 15 Sep 2017 04:05:57 +0000</resolved>
                                    <version>3.4.6</version>
                                                    <component>Performance</component>
                                        <votes>0</votes>
                                    <watches>7</watches>
                                                                                                                <comments>
                            <comment id="1673545" author="thomas.schubert" created="Fri, 15 Sep 2017 04:05:57 +0000"  >&lt;p&gt;Hi &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=dorlov&quot; class=&quot;user-hover&quot; rel=&quot;dorlov&quot;&gt;dorlov&lt;/a&gt;,&lt;/p&gt;

&lt;p&gt;Sorry for the delay getting back to you. I&apos;ve examined the diagnostic.data and believe that the behavior you&apos;re observing is expected. During checkpoints, the system becomes I/O bound and queries are impacted. Due to the nature of replication, this bottleneck may have larger impact as reads queue behind the oplog applier.&lt;/p&gt;

&lt;p&gt;From the provided data, I do not see anything to indicate a bug in the MongoDB server. For MongoDB-related support discussion please post on the &lt;a href=&quot;http://groups.google.com/group/mongodb-user&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;mongodb-user group&lt;/a&gt; or &lt;a href=&quot;http://stackoverflow.com/questions/tagged/mongodb&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;Stack Overflow with the &lt;tt&gt;mongodb&lt;/tt&gt; tag&lt;/a&gt;. A question like this involving more discussion would be best posted on the mongodb-users group.&lt;/p&gt;

&lt;p&gt;Kind regards,&lt;br/&gt;
Kelsey&lt;/p&gt;</comment>
                            <comment id="1649284" author="dorlov" created="Wed, 16 Aug 2017 07:34:33 +0000"  >&lt;p&gt;Files are uploaded&lt;/p&gt;</comment>
                            <comment id="1648865" author="thomas.schubert" created="Tue, 15 Aug 2017 18:55:07 +0000"  >&lt;p&gt;I&apos;ve created a &lt;a href=&quot;https://10gen-httpsupload.s3.amazonaws.com/upload_forms/059b3e05-6ae9-4ae0-bf47-2dc0f5b026e4.html&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;secure upload portal&lt;/a&gt; for you to use. Files uploaded to this portal are only visible to MongoDB employees investigating this issue and are routinely deleted after some time.&lt;/p&gt;</comment>
                            <comment id="1648690" author="thomas.schubert" created="Tue, 15 Aug 2017 16:00:25 +0000"  >&lt;p&gt;Hi &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=dorlov&quot; class=&quot;user-hover&quot; rel=&quot;dorlov&quot;&gt;dorlov&lt;/a&gt;,&lt;/p&gt;

&lt;p&gt;Thanks for reporting this behavior. So we can continue to investigate, would you please provide an archive of the &lt;tt&gt;diagnostic.data&lt;/tt&gt; in the &lt;tt&gt;$dbpath&lt;/tt&gt;? Please be sure to include &lt;tt&gt;diagnostic.data&lt;/tt&gt; for both primary and secondary nodes so we can compare.&lt;/p&gt;

&lt;p&gt;Thank you,&lt;br/&gt;
Thomas&lt;/p&gt;</comment>
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                            <customfieldvalue>&lt;p&gt;Works fine steps:&lt;br/&gt;
1. Setup java client with read preferences &quot;primaryPreferred&quot;&lt;br/&gt;
2. Check read performance (aprox. 2200 rps with avg. time 0.3 ms)&lt;br/&gt;
3. During fsync operation (every minute) response time dropped to 0.5 ms&lt;/p&gt;

&lt;p&gt;Change secondary and primary via rs.stepDown - time pattern is the same&lt;/p&gt;

&lt;p&gt;Works strange steps:&lt;br/&gt;
1. Setup java client with read preferences &quot;secondaryPreferred&quot;&lt;br/&gt;
2. Check read performance (aprox. 2200 rps with avg. time 0.3 ms)&lt;br/&gt;
3. During fsync operation (every minute) response time dropped to 10 ms (&lt;b&gt;20 times slower&lt;/b&gt;)&lt;/p&gt;

&lt;p&gt;Change secondary and primary via rs.stepDown - time pattern is the same&lt;/p&gt;

&lt;p&gt;Looks like secondary node works incorrect during fsync events and blocks most read operations&lt;/p&gt;</customfieldvalue>

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