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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-10503] High iowait on secondary but don&apos;t see many writes in MongoDB web interface</title>
                <link>https://jira.mongodb.org/browse/SERVER-10503</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;This server is the secondary of a replica set which is the primary of a sharding cluster. There is iowait &amp;gt;10% constantly and I see writes of about 1.5Mb/s with iotop. In the mongo web interface I don&apos;t see many writes - in fact one lists the collection as blank. &lt;/p&gt;

&lt;p&gt;Any idea why the iowait is so high? Or a way to track down which collection is causing the writes?&lt;/p&gt;</description>
                <environment>rhel6, 40GB ram, RAID5</environment>
        <key id="85884">SERVER-10503</key>
            <summary>High iowait on secondary but don&apos;t see many writes in MongoDB web interface</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="stephen.steneker@mongodb.com">Stennie Steneker</assignee>
                                    <reporter username="niccottrell">Nic Cottrell (Personal)</reporter>
                        <labels>
                    </labels>
                <created>Wed, 14 Aug 2013 08:35:05 +0000</created>
                <updated>Wed, 10 Dec 2014 23:10:25 +0000</updated>
                            <resolved>Mon, 19 Aug 2013 01:43:51 +0000</resolved>
                                    <version>2.4.5</version>
                                                                        <votes>0</votes>
                                    <watches>4</watches>
                                                                                                                <comments>
                            <comment id="405913" author="stennie" created="Mon, 19 Aug 2013 01:43:51 +0000"  >&lt;p&gt;Hi Nic,&lt;/p&gt;

&lt;p&gt;The SERVER project is for reporting bugs or feature suggestions for the MongoDB server.&lt;/p&gt;

&lt;p&gt;For MongoDB-related support discussion please post on the mongodb-users group (&lt;a href=&quot;http://groups.google.com/group/mongodb-user&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;http://groups.google.com/group/mongodb-user&lt;/a&gt;) or Stack Overflow / ServerFault.&lt;/p&gt;

&lt;p&gt;In terms of looking at current activity on your server, in addition to mongostat (which you are already using) you could use &lt;a href=&quot;http://docs.mongodb.org/manual/reference/method/db.currentOp/&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;db.currentOp()&lt;/a&gt; to report in-progress operations.  The currentOp output is the top part of the screenshot you attached from the admin interface.&lt;/p&gt;

&lt;p&gt;I had a look at this host in MMS, and it seems like your I/O is saturated.  The background flushes are consistently taking ~35s and the iostat for sdb is hovering around 6,000 writes per second.  You might be able to better distribute some of the I/O by moving the journal to a separate drive or reducing the &lt;a href=&quot;http://docs.mongodb.org/manual/reference/parameters/#param.syncdelay&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;syncdelay&lt;/a&gt; from the default of 60 seconds.  You also note that you are using RAID5; this is definitely not the best choice for performance and will have significant overhead.  As per our &lt;a href=&quot;http://docs.mongodb.org/manual/administration/production-notes/#raid&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;Linux production notes&lt;/a&gt;, RAID-10 is recommended.  It also looks like your inserts &amp;amp; queries are significantly weighted toward shard1, so you may want to reconsider your choice of shard key.&lt;/p&gt;

&lt;p&gt;For further discussion on this, I would suggest posting in one of the support forums.&lt;/p&gt;

&lt;p&gt;Regards,&lt;br/&gt;
Stephen&lt;/p&gt;</comment>
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                    <attachments>
                            <attachment id="29882" name="Screen Shot 2013-08-14 at 10.34.27 AM.png" size="248206" author="niccottrell" created="Wed, 14 Aug 2013 08:35:05 +0000"/>
                            <attachment id="29881" name="mongostat.txt" size="3817" author="niccottrell" created="Wed, 14 Aug 2013 08:35:05 +0000"/>
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