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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-13994] do not aggressively pre-fetch data for parallelCollectionScan</title>
                <link>https://jira.mongodb.org/browse/SERVER-13994</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;div class=&quot;panel&quot; style=&quot;background-color: #EEEEEE;border-color: #ccc;border-width: 1px;&quot;&gt;&lt;div class=&quot;panelHeader&quot; style=&quot;border-bottom-width: 1px;border-bottom-color: #ccc;background-color: #6CB33F;&quot;&gt;&lt;b&gt;Issue Status as of Jul 22, 2014&lt;/b&gt;&lt;/div&gt;&lt;div class=&quot;panelContent&quot; style=&quot;background-color: #EEEEEE;&quot;&gt;
&lt;p&gt;&lt;b&gt;ISSUE SUMMARY&lt;/b&gt;&lt;br/&gt;
The &lt;tt&gt;parallelCollectionScan&lt;/tt&gt; command pre-fetches the first extent for each cursor upon construction of the cursors. During cursor iteration, when a cursor needs to hop to the next extent, it also pre-fetches that extent. This can result in pre-fetching of large amounts of data. Performance tests show that disabling pre-fetching is more performant overall in most cases.&lt;/p&gt;

&lt;p&gt;&lt;b&gt;USER IMPACT&lt;/b&gt;&lt;br/&gt;
For large numbers of cursors with a big collection, this can mean aggressive pre-fetching of many gigabytes. This can cause large, unwanted spikes in disk IO.&lt;/p&gt;

&lt;p&gt;&lt;b&gt;WORKAROUNDS&lt;/b&gt;&lt;br/&gt;
None.&lt;/p&gt;

&lt;p&gt;&lt;b&gt;AFFECTED VERSIONS&lt;/b&gt;&lt;br/&gt;
All production releases up to version 2.6.3 are affected by this issue.&lt;/p&gt;

&lt;p&gt;&lt;b&gt;FIX VERSION&lt;/b&gt;&lt;br/&gt;
The fix is included in the 2.6.4 production release.&lt;/p&gt;

&lt;p&gt;&lt;b&gt;RESOLUTION DETAILS&lt;/b&gt;&lt;br/&gt;
Pre-fetching was disabled for the &lt;tt&gt;parallelCollectionScan&lt;/tt&gt; command.&lt;/p&gt;
&lt;/div&gt;&lt;/div&gt;

&lt;h6&gt;&lt;a name=&quot;Originaldescription&quot;&gt;&lt;/a&gt;Original description&lt;/h6&gt;

&lt;p&gt;The parallelCollectionScan command pre-fetches the first extent for each cursor upon construction of the cursors. During cursor iteration, when a cursor needs to hop to the next extent, it also pre-fetches that extent.&lt;/p&gt;

&lt;p&gt;For large numbers of cursors with a big collection, this can mean aggressive pre-fetching of many gigabytes. This can cause large, unwanted spikes in disk IO.&lt;/p&gt;</description>
                <environment></environment>
        <key id="137187">SERVER-13994</key>
            <summary>do not aggressively pre-fetch data for parallelCollectionScan</summary>
                <type id="4" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14710&amp;avatarType=issuetype">Improvement</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="david.storch@mongodb.com">David Storch</assignee>
                                    <reporter username="david.storch@mongodb.com">David Storch</reporter>
                        <labels>
                    </labels>
                <created>Mon, 19 May 2014 18:41:07 +0000</created>
                <updated>Mon, 11 Jul 2016 17:16:14 +0000</updated>
                            <resolved>Tue, 1 Jul 2014 21:20:17 +0000</resolved>
                                    <version>2.6.1</version>
                                    <fixVersion>2.6.4</fixVersion>
                    <fixVersion>2.7.3</fixVersion>
                                    <component>Querying</component>
                                        <votes>0</votes>
                                    <watches>4</watches>
                                                                                                                <comments>
                            <comment id="666869" author="xgen-internal-githook" created="Mon, 21 Jul 2014 22:30:09 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{u&apos;username&apos;: u&apos;dstorch&apos;, u&apos;name&apos;: u&apos;David Storch&apos;, u&apos;email&apos;: u&apos;david.storch@10gen.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-13994&quot; title=&quot;do not aggressively pre-fetch data for parallelCollectionScan&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-13994&quot;&gt;&lt;del&gt;SERVER-13994&lt;/del&gt;&lt;/a&gt; turn off pre-fetching for parallelCollectionScan command&lt;br/&gt;
Branch: v2.6&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/f46bbf5de280cd9a184fc837e887895ac1aa0057&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/f46bbf5de280cd9a184fc837e887895ac1aa0057&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="641129" author="david.storch" created="Tue, 1 Jul 2014 21:20:17 +0000"  >&lt;p&gt;Fixed in master branch by this commit:&lt;/p&gt;
&lt;p/&gt;
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					&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;Author: Mathias Stearn &amp;lt;mathias@10gen.com&amp;gt;&lt;/span&gt;&lt;/pre&gt;
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					&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;    SERVER-13630 OplogStart and ParallelCollectionScan no longer rely on storage impl&lt;/span&gt;&lt;/pre&gt;
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&lt;p/&gt;</comment>
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