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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-14323] Balancer hits config servers hard even though there are no writes</title>
                <link>https://jira.mongodb.org/browse/SERVER-14323</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;Every mongos instance starts a balancer round every few seconds. While looking for candidate chunks to move, it always reads &lt;em&gt;all&lt;/em&gt; of the chunks with the given ns from the config database, many times only to realize that the collection is in fact well balanced. This may cause a significant network load.&lt;/p&gt;</description>
                <environment></environment>
        <key id="143228">SERVER-14323</key>
            <summary>Balancer hits config servers hard even though there are no writes</summary>
                <type id="1" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14703&amp;avatarType=issuetype">Bug</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="kaloian.manassiev@mongodb.com">Kaloian Manassiev</assignee>
                                    <reporter username="jjezek">Jan Je&#158;ek</reporter>
                        <labels>
                            <label>balancer</label>
                    </labels>
                <created>Fri, 20 Jun 2014 12:22:38 +0000</created>
                <updated>Wed, 20 Jul 2016 11:14:13 +0000</updated>
                            <resolved>Wed, 20 Jul 2016 11:14:13 +0000</resolved>
                                    <version>2.6.3</version>
                    <version>3.0.12</version>
                    <version>3.2.8</version>
                    <version>3.3.8</version>
                                    <fixVersion>3.3.9</fixVersion>
                                    <component>Sharding</component>
                                        <votes>2</votes>
                                    <watches>10</watches>
                                                                                                                <comments>
                            <comment id="1329563" author="kaloian.manassiev" created="Wed, 20 Jul 2016 11:14:13 +0000"  >&lt;p&gt;With the resolution of &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-22672&quot; title=&quot;Move the sharding balancer to CSRS primary&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-22672&quot;&gt;&lt;del&gt;SERVER-22672&lt;/del&gt;&lt;/a&gt;, the sharding balancer runs only on the CSRS primary and uses a single cached version of the chunks with incremental updates.&lt;/p&gt;</comment>
                            <comment id="1118137" author="mrigal" created="Fri, 18 Dec 2015 14:29:14 +0000"  >&lt;p&gt;I can also confirm a very similar problem, with unnecessary high network load!&lt;/p&gt;</comment>
                            <comment id="647347" author="greg_10gen" created="Thu, 3 Jul 2014 14:27:18 +0000"  >&lt;p&gt;Confirmed - with chunk diffing changes, the balancer chunk reload is more expensive than it needs to be.&lt;/p&gt;</comment>
                            <comment id="631121" author="jjezek" created="Mon, 23 Jun 2014 09:28:35 +0000"  >&lt;p&gt;This is the query that causes the network load:&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/blob/v2.6.3/src/mongo/s/balance.cpp#L280&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/blob/v2.6.3/src/mongo/s/balance.cpp#L280&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="631118" author="jjezek" created="Mon, 23 Jun 2014 09:22:56 +0000"  >&lt;p&gt;Sorry about the version confusion. I have looked at the source code to confirm my assumptions and that was in the v2.6.3 branch. In production we actually use a 2.4 that comes with Debian.&lt;br/&gt;
I have spawned a new temporary mongos for the cluster with a -vvvvv parameter. The log is attached. It is run against a similar cluster running in our staging environment, which is a bit smaller than production, nonetheless the problem is also visible.&lt;/p&gt;</comment>
                            <comment id="631099" author="jjezek" created="Mon, 23 Jun 2014 09:02:10 +0000"  >&lt;p&gt;Log from a temporary mongos process run with -vvvvv&lt;/p&gt;</comment>
                            <comment id="629023" author="thomasr" created="Fri, 20 Jun 2014 14:51:09 +0000"  >&lt;p&gt;Hi Jan,&lt;/p&gt;

&lt;p&gt;Can you please attach the log file of the mongos that is doing the balancing? &lt;/p&gt;

&lt;p&gt;Also just to confirm: Version 2.6.3 has not been released yet. Are you running 2.6.2?&lt;/p&gt;

&lt;p&gt;Thanks,&lt;br/&gt;
Thomas&lt;/p&gt;
</comment>
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                                        <issuelink>
            <issuekey id="265925">SERVER-22672</issuekey>
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                            <issuelinktype id="10010">
                    <name>Duplicate</name>
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            <issuekey id="151161">SERVER-14810</issuekey>
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                            <attachment id="46245" name="mongos.log.gz" size="6849" author="jjezek" created="Mon, 23 Jun 2014 09:02:10 +0000"/>
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                            <customfieldvalue>&lt;p&gt;In my case, I have a database in 2 shards with slightly less than 30M documents in about 130k chunks. I have 6 mongos instances and, each reading from the first config server at about 20 mbit/s which is 120mbit/s in total. I have to control the balancer manually due to this.&lt;/p&gt;</customfieldvalue>

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