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    <title>MongoDB Jira</title>
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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-62099] POC balancer must take decisions based on collection sizes on shards</title>
                <link>https://jira.mongodb.org/browse/SERVER-62099</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;ul class=&quot;alternate&quot; type=&quot;square&quot;&gt;
	&lt;li&gt;Make the balancer call dataSize on shards to understand which are the most/least full.&lt;/li&gt;
	&lt;li&gt;Implement &#8220;dummy&#8221; donateData command (calling into move chunk) to move data between shards according to the their filling rate.&lt;/li&gt;
&lt;/ul&gt;
</description>
                <environment></environment>
        <key id="1952365">SERVER-62099</key>
            <summary>POC balancer must take decisions based on collection sizes on shards</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>
                    </labels>
                <created>Thu, 16 Dec 2021 17:23:24 +0000</created>
                <updated>Thu, 20 Jan 2022 15:10:14 +0000</updated>
                            <resolved>Thu, 20 Jan 2022 15:09:50 +0000</resolved>
                                                                    <component>Sharding</component>
                                        <votes>0</votes>
                                    <watches>1</watches>
                                                                                                                <comments>
                            <comment id="4302933" author="pierlauro.sciarelli" created="Thu, 20 Jan 2022 15:09:50 +0000"  >&lt;p&gt;Outcome of the &lt;span class=&quot;error&quot;&gt;&amp;#91;inefficient &amp;amp; dirty-coded&amp;#93;&lt;/span&gt; POC:&lt;/p&gt;
&lt;ul class=&quot;alternate&quot; type=&quot;square&quot;&gt;
	&lt;li&gt;It was possible to modify the balancer in order to take into account the data size on shards rather than the number of chunks (with an additional broadcast to all shards at the beginning of each round).&lt;/li&gt;
	&lt;li&gt;Implemented &lt;tt&gt;donateData&lt;/tt&gt; command received from shards with parameters &lt;tt&gt;(namespace, toShard, minBound)&lt;/tt&gt; that - at moment - simply calls into &lt;tt&gt;moveChunk&lt;/tt&gt; to move the chunk with &lt;tt&gt;minBound&lt;/tt&gt;.&lt;/li&gt;
	&lt;li&gt;The &lt;tt&gt;moveChunk&lt;/tt&gt; call triggered in balancing rounds has been replaced with a call to &lt;tt&gt;donateData&lt;/tt&gt;&lt;/li&gt;
&lt;/ul&gt;


&lt;p&gt;Relevant problems discovered while POC-ing:&lt;/p&gt;
&lt;ul class=&quot;alternate&quot; type=&quot;square&quot;&gt;
	&lt;li&gt;Since &lt;tt&gt;dataSize&lt;/tt&gt; includes orphans, in order for the balancer to work properly we need to track orphans somehow . For the POC, instead of calling &lt;tt&gt;dataSize&lt;/tt&gt; the balancer is relying on &lt;tt&gt;countDocuments&lt;/tt&gt;, but that is obviously not a viable  solution because it means performing a whole index scan for every collection at every round.&lt;/li&gt;
&lt;/ul&gt;
</comment>
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