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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-82216] Make a more even distribution of chunks through shards for initial split policies with zones</title>
                <link>https://jira.mongodb.org/browse/SERVER-82216</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;Choosing the correct strategy to initialize chunks to the shards is important to have an even distribution, i.e. all shards have the same amount of chunks.&lt;/p&gt;

&lt;p&gt;Today&#8217;s &lt;a href=&quot;https://github.com/mongodb/mongo/blob/62b3b9a2d52afeae975c9c6dbb0430c77b72a055/src/mongo/db/s/config/initial_split_policy.cpp#L488&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;createFirstChunks&lt;/a&gt; for initial split policies with zones will first create one (or some) chunk per zone and fill the gaps not contained in the zone&#8217;s domain. The current implementation distributes the chunks already created starting from the MinKey to the MaxKey, using a round-robin strategy. Because of zones can enforce chunks to be placed into specific shards, we could end up having some shards with more chunks than others.&lt;/p&gt;

&lt;p&gt;An example of that situation could be:&lt;/p&gt;

&lt;p&gt;Cluster with three shards (Shard0, Shard1, Shard2):&lt;/p&gt;
&lt;ul&gt;
	&lt;li&gt;presplitHashedZones = true&lt;/li&gt;
	&lt;li&gt;zoneA for Shard0 with {-10, 0}&lt;/li&gt;
	&lt;li&gt;zoneB for Shard0 with {0, MaxKey}&lt;/li&gt;
	&lt;li&gt;numInitialChunks = default (a.k.a 2)&lt;/li&gt;
&lt;/ul&gt;


&lt;p&gt;Chunk Distribution:&lt;/p&gt;
&lt;ul&gt;
	&lt;li&gt;Shard0: chunkA{MinKey, -10}, chunkB{-10, 0}, chunkC{0, MaxKey},&lt;/li&gt;
	&lt;li&gt;Shard1: none&lt;/li&gt;
	&lt;li&gt;Shard2: none&lt;/li&gt;
&lt;/ul&gt;


&lt;p&gt;I propose improving this algorithm with first distributing all chunks associated with a zone, and then fill the gaps according to the number of chunks per shard.&lt;/p&gt;</description>
                <environment></environment>
        <key id="2475296">SERVER-82216</key>
            <summary>Make a more even distribution of chunks through shards for initial split policies with zones</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="10038" iconUrl="https://jira.mongodb.org/images/icons/subtask.gif" description="">Backlog</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="pol.pinol@mongodb.com">Pol Pinol</reporter>
                        <labels>
                            <label>car-qw</label>
                            <label>oldshardingemea</label>
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                <created>Mon, 16 Oct 2023 14:29:20 +0000</created>
                <updated>Thu, 26 Oct 2023 09:57:55 +0000</updated>
                                                                                                <votes>0</votes>
                                    <watches>1</watches>
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            <issuekey id="2320797">SERVER-76405</issuekey>
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