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        <build-date>13-04-2023</build-date>
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<item>
            <title>[SERVER-25249] High I/O load in secondary nodes with WiredTiger engine (caused by journaling)</title>
                <link>https://jira.mongodb.org/browse/SERVER-25249</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;Hi,&lt;br/&gt;
  I upgraded my mongodb cluster from 3.0 to 3.2.7 recently and found that the I/O load increased a lot. (abount 100% in primary nodes and 500% in secondary nodes, mainly judged by %util of iostat command).&lt;br/&gt;
  After reading the doc, I learned that journaling behavior changed a little in 3.2 (flushes the journal every 50ms), so I tried disabling the journaling and the I/O load returned low as it was in 3.0.  So I guess flushing the journal frequently is the main reason of primary nodes&apos; I/O load going high 100%. &lt;br/&gt;
  But I can&apos;t find out why secondaries&apos; I/O load increased 500% in the docs. So I used strace to track the mongod thread which is in charge of flushing the journal. (strace about 10 seconds)&lt;/p&gt;

&lt;p&gt;Primary node:&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;
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				&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;% time     seconds  usecs/call     calls    errors syscall&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;------ ----------- ----------- --------- --------- ----------------&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; 58.86    3.921470      13476       291       143   futex&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; 40.54    2.700511      18754       144             pwrite&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
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				&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;  0.30    0.020001      10001         2             fdatasync&lt;/span&gt;&lt;/pre&gt;
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&lt;p/&gt;

&lt;p&gt;Secondary node:&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;
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					&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;% time     seconds  usecs/call     calls    errors syscall&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
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				&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;------ ----------- ----------- --------- --------- ----------------&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
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				&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; 83.04    4.272435       14682       291             fdatasync&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
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				&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; 16.95    0.871993         998       874         9   futex&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;  0.01    0.000461           2       288             pwrite&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;  From the above we know that pwrite calls in secondary node are nearly twice of primary. And fdatasync calls are as many as pwrite calls and are far more than primary&apos;s. Is this the reason why secondaries&apos; I/O load increase 500% ?  Is it a bug or a design?&lt;/p&gt;</description>
                <environment></environment>
        <key id="303673">SERVER-25249</key>
            <summary>High I/O load in secondary nodes with WiredTiger engine (caused by journaling)</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="kelsey.schubert@mongodb.com">Kelsey Schubert</assignee>
                                    <reporter username="strlee">stronglee</reporter>
                        <labels>
                            <label>RF</label>
                    </labels>
                <created>Mon, 25 Jul 2016 08:39:25 +0000</created>
                <updated>Fri, 8 Jan 2021 19:46:39 +0000</updated>
                            <resolved>Fri, 19 Aug 2016 17:05:03 +0000</resolved>
                                    <version>3.2.7</version>
                                                    <component>WiredTiger</component>
                                        <votes>0</votes>
                                    <watches>12</watches>
                                                                                                                <comments>
                            <comment id="1362276" author="thomas.schubert" created="Fri, 19 Aug 2016 17:05:03 +0000"  >&lt;p&gt;Hi &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=strlee&quot; class=&quot;user-hover&quot; rel=&quot;strlee&quot;&gt;strlee&lt;/a&gt;,&lt;/p&gt;

&lt;p&gt;Thanks for the confirmation. We expect that that secondary would self-throttle if there was additional I/O load present.&lt;/p&gt;

&lt;p&gt;If secondaries are falling behind and you do not see the secondaries self-throttle, please open a new ticket and we will investigate.&lt;/p&gt;

&lt;p&gt;Kind regards,&lt;br/&gt;
Thomas&lt;/p&gt;</comment>
                            <comment id="1361760" author="strlee" created="Fri, 19 Aug 2016 04:49:37 +0000"  >&lt;p&gt;Hi Thomas Schubert,&lt;br/&gt;
  You are right, there are no obvious adverse affects on my secondaries except for the increased I/O load.&lt;br/&gt;
But the reason may be that I don&apos;t have heavy query jobs on my secondaries. I wonder if when I have heavy&lt;br/&gt;
jobs on them, the performance would be poor because of the high I/O load.&lt;br/&gt;
  Is it possible to make the secondaries sync the journal in a configrable interval to lower to I/O load? I think &lt;br/&gt;
not all systems need so high reliability. e.g. It is no problem for my system to lose data for a few seconds.&lt;/p&gt;</comment>
                            <comment id="1361229" author="thomas.schubert" created="Thu, 18 Aug 2016 18:21:33 +0000"  >&lt;p&gt;Hi &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=strlee&quot; class=&quot;user-hover&quot; rel=&quot;strlee&quot;&gt;strlee&lt;/a&gt;,&lt;/p&gt;

&lt;p&gt;Thanks for the detailed report of this behavior. It is by design that replication on secondaries will sync the journal faster than it would for the same load on a primary. The syncing is done asynchronously, and so it should not affect replication throughput unless the disk is saturated.&lt;/p&gt;

&lt;p&gt;My understanding is that you are not observing any adverse affects as a result of the increased I/O load on the secondaries. Is this correct?&lt;/p&gt;

&lt;p&gt;Kind regards,&lt;br/&gt;
Thomas&lt;/p&gt;</comment>
                            <comment id="1337146" author="strlee" created="Wed, 27 Jul 2016 04:23:29 +0000"  >&lt;p&gt;I tried set journalCommitInterval=500, the pwrite/fdatasync calls didn&apos;t go down too much (approximately from 300 to 200).&lt;br/&gt;
I also tried set cfg.protocolVersion=0, nothing happened.&lt;br/&gt;
I don&apos;t want to disable journaling, but the I/O load increased so much. &lt;br/&gt;
Is there a way to let mongodb flush journal in a certain interval?&lt;/p&gt;</comment>
                    </comments>
                <issuelinks>
                            <issuelinktype id="10012">
                    <name>Related</name>
                                            <outwardlinks description="related to">
                                        <issuelink>
            <issuekey id="1584556">SERVER-53667</issuekey>
        </issuelink>
                            </outwardlinks>
                                                                <inwardlinks description="is related to">
                                        <issuelink>
            <issuekey id="315179">SERVER-26040</issuekey>
        </issuelink>
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