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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-60267] Change jumbo chunk size threshold</title>
                <link>https://jira.mongodb.org/browse/SERVER-60267</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;Review &lt;a href=&quot;https://github.com/mongodb/mongo/blob/378f22798d19eddb1db8e53a55c2a90c32d23ae3/src/mongo/db/s/config/sharding_catalog_manager_chunk_operations.cpp#L1458-L1471&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;the logic&lt;/a&gt; that is used to mark chunks as jumbo to see if it still makes sense after the changes introduced in &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-60009&quot; title=&quot;Increase auto splitted chunks size&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-60009&quot;&gt;&lt;del&gt;SERVER-60009&lt;/del&gt;&lt;/a&gt; that added the ability to keep chunks greater than &lt;tt&gt;maxChunkSize&lt;/tt&gt; until some controlled threshold.&lt;/p&gt;

&lt;p&gt;&lt;span class=&quot;error&quot;&gt;&amp;#91;EDIT&amp;#93;&lt;/span&gt; Chosen solution: mark a chunk as jumbo if its size is at least &lt;tt&gt;2 * maxChunkSize&lt;/tt&gt; (aka double &lt;a href=&quot;https://github.com/mongodb/mongo/blob/524e36a47726dfdde7b99175d2a6e235e063fabe/src/mongo/db/s/migration_chunk_cloner_source_legacy.cpp#L894&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;maxRecsWhenFull&lt;/a&gt;).&lt;/p&gt;</description>
                <environment></environment>
        <key id="1885148">SERVER-60267</key>
            <summary>Change jumbo chunk size threshold</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="13201">Fixed</resolution>
                                        <assignee username="pierlauro.sciarelli@mongodb.com">Pierlauro Sciarelli</assignee>
                                    <reporter username="pierlauro.sciarelli@mongodb.com">Pierlauro Sciarelli</reporter>
                        <labels>
                            <label>PM-2321-Chunk-Splitter</label>
                    </labels>
                <created>Tue, 28 Sep 2021 09:02:44 +0000</created>
                <updated>Sun, 29 Oct 2023 21:48:07 +0000</updated>
                            <resolved>Tue, 5 Oct 2021 09:27:37 +0000</resolved>
                                                    <fixVersion>5.1.0-rc0</fixVersion>
                                                        <votes>0</votes>
                                    <watches>2</watches>
                                                                                                                <comments>
                            <comment id="4535451" author="pierlauro.sciarelli" created="Mon, 9 May 2022 15:23:09 +0000"  >&lt;p&gt;Hi &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=jean.coulon%40mongodb.com&quot; class=&quot;user-hover&quot; rel=&quot;jean.coulon@mongodb.com&quot;&gt;jean.coulon@mongodb.com&lt;/a&gt;,&lt;br/&gt;
the change has also been backported to all currently supported version under the &lt;a href=&quot;https://jira.mongodb.org/issues/?jql=text%20~%20%22%5C%22backport%20the%20autoSplitVector%20logic%5C%22%22%20and%20project%20%3D%20%22Core%20Server%22%20&quot; class=&quot;external-link&quot; rel=&quot;nofollow&quot;&gt;following tickets&lt;/a&gt; to backport all the new auto splitter changes (including this) in one shot. Opened &lt;a href=&quot;https://jira.mongodb.org/browse/DOCS-15329&quot; title=&quot;Document change of jumbo size treshold&quot; class=&quot;issue-link&quot; data-issue-key=&quot;DOCS-15329&quot;&gt;DOCS-15329&lt;/a&gt; to document this change (I was not aware the threshold was so precisely documented), thanks for pointing that out.&lt;/p&gt;</comment>
                            <comment id="4535375" author="JIRAUSER1265233" created="Mon, 9 May 2022 15:08:29 +0000"  >&lt;p&gt;Hello &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=pierlauro.sciarelli%40mongodb.com&quot; class=&quot;user-hover&quot; rel=&quot;pierlauro.sciarelli@mongodb.com&quot;&gt;pierlauro.sciarelli@mongodb.com&lt;/a&gt; ,&#160;&lt;/p&gt;

&lt;p&gt;Small question here : is this change supposed to be live in other versions than &lt;a href=&quot;https://jira.mongodb.org/issues/?jql=project+%3D+SERVER+AND+fixVersion+%3D+5.1.0-rc0&quot; class=&quot;external-link&quot; rel=&quot;nofollow&quot;&gt;5.1.0-rc0&lt;/a&gt; ?&#160;&lt;/p&gt;

&lt;p&gt;I have a customer in 4.4.13 and it seems that the threshold has been changed from 1.3 to 2.&#160;&lt;/p&gt;

&lt;p&gt;From documentation (&lt;a href=&quot;https://www.mongodb.com/docs/manual/core/sharding-balancer-administration/&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://www.mongodb.com/docs/manual/core/sharding-balancer-administration/&lt;/a&gt;) :&lt;/p&gt;
&lt;blockquote&gt;&lt;p&gt;By default, MongoDB cannot move a chunk if the number of documents in the chunk is greater than 1.3 times the result of dividing the configured chunk size by the average document size. db.collection.stats() includes the avgObjSize field, which represents the average document size in the collection.&lt;/p&gt;&lt;/blockquote&gt;
&lt;p&gt;From log we see :&lt;/p&gt;
&lt;blockquote&gt;&lt;p&gt;Cannot move chunk: the maximum number of documents for a chunk is 618, the maximum chunk size is 67108864, average document size is 217075. Found 3375 documents in chunk&lt;/p&gt;&lt;/blockquote&gt;
&lt;p&gt;configuredchunksize : 67108864&lt;/p&gt;

&lt;p&gt;average document size : 217075&lt;/p&gt;

&lt;p&gt;So according to previous formula, maximum number of documents should be equal to :&lt;/p&gt;

&lt;p&gt;1,3 * (67108864/217075) = 402&lt;/p&gt;

&lt;p&gt;However log told us :&lt;/p&gt;
&lt;blockquote&gt;&lt;p&gt;the maximum number of documents for a chunk is 618&lt;/p&gt;&lt;/blockquote&gt;
&lt;p&gt;So :&lt;/p&gt;

&lt;p&gt;2 * (67108864/217075) = 618&lt;/p&gt;

&lt;p&gt;Now we are matching the value displayed in log.&lt;/p&gt;

&lt;p&gt;According to &lt;a href=&quot;https://github.com/mongodb/mongo/blob/0b7b26390896197c0cb4f31affcda7dc19dda85f/src/mongo/db/s/migration_chunk_cloner_source_legacy.cpp&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;source code&lt;/a&gt; :&#160;&lt;/p&gt;
&lt;blockquote&gt;&lt;p&gt;maxRecsWhenFull = 2 * maxRecsWhenFull; // pad some slack&lt;/p&gt;&lt;/blockquote&gt;
&lt;p&gt;If this has been changed in all versions, we should change the documentation (DOCSP-22524)&lt;/p&gt;

&lt;p&gt;&#160;&lt;/p&gt;</comment>
                            <comment id="4106419" author="JIRAUSER1259052" created="Wed, 6 Oct 2021 17:24:34 +0000"  >&lt;p&gt;Updating the fixversion since branching activities occurred yesterday. This ticket will be in rc0 when it&#8217;s been triggered. For more active release information, please keep an eye on #server-release. Thank you!&lt;/p&gt;</comment>
                            <comment id="4101728" author="xgen-internal-githook" created="Tue, 5 Oct 2021 09:23:37 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;Pierlauro Sciarelli&apos;, &apos;email&apos;: &apos;pierlauro.sciarelli@mongodb.com&apos;, &apos;username&apos;: &apos;pierlauro&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-60267&quot; title=&quot;Change jumbo chunk size threshold&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-60267&quot;&gt;&lt;del&gt;SERVER-60267&lt;/del&gt;&lt;/a&gt; Change jumbo chunk size threshold&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/033e43fe45b29c5d6658b0dc1e1d53b8cc50aecc&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/033e43fe45b29c5d6658b0dc1e1d53b8cc50aecc&lt;/a&gt;&lt;/p&gt;</comment>
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