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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-72666] The presplit optimisation of shardCollection will always assign a chunk to the primary shard</title>
                <link>https://jira.mongodb.org/browse/SERVER-72666</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;When &lt;tt&gt;shardCollection&lt;/tt&gt; is invoked on an empty collection specifying a hashed key, the DDL will perform an optimisation: it will split its key space in a number of chunks (using the &lt;tt&gt;numInitialChunks&lt;/tt&gt; parameter) and then distribute them across different shards; by this way, the collection will be added to the sharding catalog as already balanced.&lt;/p&gt;

&lt;p&gt;The distribution is not random: instead, chunks will be assigned to shards &lt;a href=&quot;https://github.com/mongodb/mongo/blob/971761c5670303bad903da214f20077dcb484e69/src/mongo/db/s/config/initial_split_policy.cpp#L374-L375&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;following the alphabetical order of shard IDs&lt;/a&gt; (except for the primary shard, that is assured to &lt;a href=&quot;https://github.com/mongodb/mongo/blob/971761c5670303bad903da214f20077dcb484e69/src/mongo/db/s/config/initial_split_policy.cpp#L247-L249&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;always receive at least one chunk&lt;/a&gt;).&lt;/p&gt;

&lt;p&gt;Such strategy may lead to a state of data imbalance at cluster level when:&lt;/p&gt;
&lt;ul&gt;
	&lt;li&gt;there is a high number of empty collections being sharded with a hashed key&lt;/li&gt;
	&lt;li&gt;the value of &lt;tt&gt;numInitialChunks&lt;/tt&gt; is lower than the number of shards in the cluster.&lt;/li&gt;
&lt;/ul&gt;


&lt;p&gt;In order to mitigate such effect (in addition to what documented in &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-72650&quot; title=&quot;shardCollection with hashed key does not randomly distribute chunks across the cluster&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-72650&quot;&gt;&lt;del&gt;SERVER-72650&lt;/del&gt;&lt;/a&gt;) the constraint of always having at least one chunk in the primary shard of the collection being sharded could be removed. &lt;/p&gt;</description>
                <environment></environment>
        <key id="2228740">SERVER-72666</key>
            <summary>The presplit optimisation of shardCollection will always assign a chunk to the primary shard</summary>
                <type id="4" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14710&amp;avatarType=issuetype">Improvement</type>
                                            <priority id="3" iconUrl="https://jira.mongodb.org/images/icons/priorities/major.svg">Major - P3</priority>
                        <status id="1" iconUrl="https://jira.mongodb.org/images/icons/statuses/open.png" description="">Open</status>
                    <statusCategory id="2" key="new" colorName="default"/>
                                    <resolution id="-1">Unresolved</resolution>
                                        <assignee username="backlog-server-catalog-and-routing">Backlog - Catalog and Routing</assignee>
                                    <reporter username="paolo.polato@mongodb.com">Paolo Polato</reporter>
                        <labels>
                            <label>oldshardingemea</label>
                            <label>shardingemea-qw</label>
                    </labels>
                <created>Tue, 10 Jan 2023 11:37:44 +0000</created>
                <updated>Thu, 26 Oct 2023 09:54:40 +0000</updated>
                                                                                                <votes>0</votes>
                                    <watches>2</watches>
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