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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>[DOCS-9441] Docs for SERVER-26579: Sharding balancer moves chunks to shards whose utilization is above the cluster average</title>
                <link>https://jira.mongodb.org/browse/DOCS-9441</link>
                <project id="10380" key="DOCS">Documentation</project>
                    <description>&lt;p&gt;Engineering Ticket Description:&lt;/p&gt;

&lt;p&gt;The current sharding balancer policy attempts to move chunks from shards which have more than the cluster-wide per-shard chunk count average to shards which have less than it. However there is a bug due to which chunks will move from highly loaded to less-loaded shard if the difference between the two is sufficiently large.&lt;/p&gt;

&lt;p&gt;In particular consider the following case:&lt;/p&gt;
&lt;p/&gt;
&lt;div id=&quot;syntaxplugin&quot; class=&quot;syntaxplugin&quot; style=&quot;border: 1px dashed #bbb; border-radius: 5px !important; overflow: auto; max-height: 30em;&quot;&gt;
&lt;table cellspacing=&quot;0&quot; cellpadding=&quot;0&quot; border=&quot;0&quot; width=&quot;100%&quot; style=&quot;font-size: 1em; line-height: 1.4em !important; font-weight: normal; font-style: normal; color: black;&quot;&gt;
		&lt;tbody &gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;  margin-top: 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;Shard1 - 1000&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;Shard2 - 900&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;Shard3 - 900&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;Shard4 - 800&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;Shard5 - 0&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   margin-bottom: 10px;  width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;Shard6 - 0&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
			&lt;/tbody&gt;
&lt;/table&gt;
&lt;/div&gt;
&lt;p/&gt;

&lt;p&gt;Intuitively, this should produce only two migrations: Shard1 -&amp;gt; Shard5 and Shard2 -&amp;gt; Shard 6. However it will also cause a third migration from Shard3 -&amp;gt; Shard4, because 900 is above the ideal per-shard chunk number of 600.&lt;/p&gt;

&lt;p&gt;Via &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=kaloian.manassiev&quot; class=&quot;user-hover&quot; rel=&quot;kaloian.manassiev&quot;&gt;kaloian.manassiev&lt;/a&gt; &lt;br/&gt;
&amp;gt; With this change, the balancer will now also attempt to move chunks out of shards whose max size setting is exceeded.&lt;/p&gt;</description>
                <environment></environment>
        <key id="335949">DOCS-9441</key>
            <summary>Docs for SERVER-26579: Sharding balancer moves chunks to shards whose utilization is above the cluster average</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="12300">Won&apos;t Do</resolution>
                                        <assignee username="ravind.kumar">Ravind Kumar</assignee>
                                    <reporter username="emily.hall">Emily Hall</reporter>
                        <labels>
                            <label>DF</label>
                    </labels>
                <created>Fri, 2 Dec 2016 18:17:54 +0000</created>
                <updated>Mon, 30 Oct 2023 21:21:31 +0000</updated>
                            <resolved>Fri, 17 Mar 2023 02:08:59 +0000</resolved>
                                                    <fixVersion>Server_Docs_20231030</fixVersion>
                                    <component>Server</component>
                        <due></due>
                            <votes>0</votes>
                                    <watches>3</watches>
                                                                                                                <comments>
                            <comment id="5279653" author="JIRAUSER1264463" created="Fri, 17 Mar 2023 02:08:59 +0000"  >&lt;p&gt;Closing this out on the grounds that:&lt;/p&gt;
&lt;ul&gt;
	&lt;li&gt;Details of this request are significantly outdated.&#160;&lt;/li&gt;
	&lt;li&gt;Our documentation has evolved substantially since this request was made.&#160;&lt;/li&gt;
&lt;/ul&gt;


&lt;p&gt;Based on this, closing as WON&apos;T DO. Please don&apos;t hesitate to give me a shout or to reopen if you disagree.&lt;/p&gt;</comment>
                            <comment id="1467373" author="kaloian.manassiev" created="Sat, 31 Dec 2016 13:04:29 +0000"  >&lt;p&gt;The balancer has no means to look into the &quot;read/write utilization&quot; - it only deals with number of chunks and estimated size of the chunk, using the formula that you pointed out. With &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-26579&quot; title=&quot;Sharding balancer moves chunks to shards whose utilization is above the cluster average&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-26579&quot;&gt;&lt;del&gt;SERVER-26579&lt;/del&gt;&lt;/a&gt; the balancer will move chunks out of shards, whose number of chunks is higher than the average for the cluster / number of shards. Please have a look at the example in the ticket description.&lt;/p&gt;

&lt;p&gt;The second bulled above is correct.&lt;/p&gt;</comment>
                            <comment id="1448063" author="ravind.kumar" created="Fri, 2 Dec 2016 18:39:15 +0000"  >&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=kaloian.manassiev&quot; class=&quot;user-hover&quot; rel=&quot;kaloian.manassiev&quot;&gt;kaloian.manassiev&lt;/a&gt; I don&apos;t quite follow what has been resolved here.&lt;/p&gt;

&lt;p&gt;From the summary and your comment, I think the change overall is that the balancer can move chunks off of a shard if either&lt;/p&gt;

&lt;ul&gt;
	&lt;li&gt;The chunk has excessive / greater utilization (reads/writes?), moving it to a shard with less utilization overall&lt;/li&gt;
&lt;/ul&gt;


&lt;ul&gt;
	&lt;li&gt;The number of chunks * chunksize &amp;gt; maxSize, so the balancer starts moving chunks until the shard size no longer validates maxSize.&lt;/li&gt;
&lt;/ul&gt;
</comment>
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                            <issuelinktype id="10320">
                    <name>Documented</name>
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