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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-57309] Investigate update chunks performance during setFCV 4.4 &#171;--&#187; 5.0</title>
                <link>https://jira.mongodb.org/browse/SERVER-57309</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;Given the multi-update performance regression showed in &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-56509&quot; title=&quot;Wrap unique index insertion _keyExists call in a WT cursor reconfigure.&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-56509&quot;&gt;&lt;del&gt;SERVER-56509&lt;/del&gt;&lt;/a&gt; , investigate the performance of chunks update as part of &lt;tt&gt;setFCV v4.4 &#171;--&#187; v5.0&lt;/tt&gt; and eventual fixes in case slowness is detected.&lt;/p&gt;</description>
                <environment></environment>
        <key id="1764061">SERVER-57309</key>
            <summary>Investigate update chunks performance during setFCV 4.4 &#171;--&#187; 5.0</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="9">Done</resolution>
                                        <assignee username="pierlauro.sciarelli@mongodb.com">Pierlauro Sciarelli</assignee>
                                    <reporter username="pierlauro.sciarelli@mongodb.com">Pierlauro Sciarelli</reporter>
                        <labels>
                            <label>PM-1965-Cleanup</label>
                    </labels>
                <created>Mon, 31 May 2021 10:24:09 +0000</created>
                <updated>Mon, 7 Jun 2021 12:24:39 +0000</updated>
                            <resolved>Mon, 7 Jun 2021 12:24:39 +0000</resolved>
                                                                    <component>Sharding</component>
                                        <votes>0</votes>
                                    <watches>2</watches>
                                                                                                                <comments>
                            <comment id="3862071" author="pierlauro.sciarelli" created="Mon, 7 Jun 2021 12:24:39 +0000"  >&lt;p&gt;Closing this investigation ticket and opening &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-57487&quot; title=&quot;Update chunks during setFCV 4.4 &#171;--&#187; 5.0 must not block balancing for long periods&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-57487&quot;&gt;&lt;del&gt;SERVER-57487&lt;/del&gt;&lt;/a&gt; summarizing the performance problem.&lt;/p&gt;</comment>
                            <comment id="3856886" author="pierlauro.sciarelli" created="Thu, 3 Jun 2021 09:59:40 +0000"  >&lt;p&gt;Further notes: sys-perf machines were showing a speed up of ~4x compared to the workstation during testing for &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-57307&quot; title=&quot;Improve deletion of config.chunks entries with hint and retries&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-57307&quot;&gt;&lt;del&gt;SERVER-57307&lt;/del&gt;&lt;/a&gt;, so the results reported in the table above may be considered as worst case scenario on decent machines.&lt;/p&gt;

&lt;p&gt;All the mentioned updates are happening under the chunks lock, meaning that merge/move/split operations are blocked. A possible quick improvement to reduce the window in which balancing is not allowed would be to perform the &lt;tt&gt;$unset&lt;/tt&gt; operations outside such lock.&lt;/p&gt;</comment>
                            <comment id="3853510" author="pierlauro.sciarelli" created="Wed, 2 Jun 2021 13:16:34 +0000"  >&lt;p&gt;When updating &lt;tt&gt;config.chunk&lt;/tt&gt; entries during &lt;tt&gt;setFCV&lt;/tt&gt;, the scaling is linear.&lt;/p&gt;

&lt;p&gt;Following, the results of some simple downgrade/upgrade experiments with different number of chunks (executed on the virtual workstation).&lt;/p&gt;
&lt;div class=&apos;table-wrap&apos;&gt;
&lt;table class=&apos;confluenceTable&apos;&gt;&lt;tbody&gt;
&lt;tr&gt;
&lt;th class=&apos;confluenceTh&apos;&gt;&#160;&lt;/th&gt;
&lt;th class=&apos;confluenceTh&apos;&gt;downgrade&lt;/th&gt;
&lt;th class=&apos;confluenceTh&apos;&gt;downgrade&lt;/th&gt;
&lt;th class=&apos;confluenceTh&apos;&gt;upgrade&lt;/th&gt;
&lt;th class=&apos;confluenceTh&apos;&gt;upgrade&lt;/th&gt;
&lt;/tr&gt;
&lt;tr&gt;
&lt;th class=&apos;confluenceTh&apos;&gt;number of chunks&lt;/th&gt;
&lt;th class=&apos;confluenceTh&apos;&gt;&lt;tt&gt;$set ns&lt;/tt&gt;&lt;/th&gt;
&lt;th class=&apos;confluenceTh&apos;&gt;&lt;tt&gt;$unset uuid &amp;amp; timestamp&lt;/tt&gt;&lt;/th&gt;
&lt;th class=&apos;confluenceTh&apos;&gt;&lt;tt&gt;$set uuid &amp;amp; timestamp&lt;/tt&gt;&lt;/th&gt;
&lt;th class=&apos;confluenceTh&apos;&gt;&lt;tt&gt;$unset ns&lt;/tt&gt;&lt;/th&gt;
&lt;/tr&gt;
&lt;tr&gt;
&lt;td class=&apos;confluenceTd&apos;&gt;100K&lt;/td&gt;
&lt;td class=&apos;confluenceTd&apos;&gt;32 s&lt;/td&gt;
&lt;td class=&apos;confluenceTd&apos;&gt;27 s&lt;/td&gt;
&lt;td class=&apos;confluenceTd&apos;&gt;22 s&lt;/td&gt;
&lt;td class=&apos;confluenceTd&apos;&gt;31 s&lt;/td&gt;
&lt;/tr&gt;
&lt;tr&gt;
&lt;td class=&apos;confluenceTd&apos;&gt;500K&lt;/td&gt;
&lt;td class=&apos;confluenceTd&apos;&gt;180 s&lt;/td&gt;
&lt;td class=&apos;confluenceTd&apos;&gt;164 s&lt;/td&gt;
&lt;td class=&apos;confluenceTd&apos;&gt;117 s&lt;/td&gt;
&lt;td class=&apos;confluenceTd&apos;&gt;144 s&lt;/td&gt;
&lt;/tr&gt;
&lt;tr&gt;
&lt;td class=&apos;confluenceTd&apos;&gt;1M&lt;/td&gt;
&lt;td class=&apos;confluenceTd&apos;&gt;324 s&lt;/td&gt;
&lt;td class=&apos;confluenceTd&apos;&gt;313 s&lt;/td&gt;
&lt;td class=&apos;confluenceTd&apos;&gt;211 s&lt;/td&gt;
&lt;td class=&apos;confluenceTd&apos;&gt;271 s&lt;/td&gt;
&lt;/tr&gt;
&lt;/tbody&gt;&lt;/table&gt;
&lt;/div&gt;


&lt;p&gt;The current order in which indexes are created/dropped on the added/removed fields also turns out to be optimal. For example, the &lt;tt&gt;uuid&lt;/tt&gt; field is added before creating the 3 secondary indexes on it and this ensures that &lt;tt&gt;$set&lt;/tt&gt; operations don&apos;t need to update all the indexes. Similarly, on downgrade the &lt;tt&gt;ns&lt;/tt&gt; indexes are dropped before actually removing the &lt;tt&gt;ns&lt;/tt&gt; field from entries.&lt;/p&gt;</comment>
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