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    <description>This file is an XML representation of an issue</description>
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        <version>9.7.1</version>
        <build-number>970001</build-number>
        <build-date>13-04-2023</build-date>
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<item>
            <title>[DOCS-14071] Remove all references to &quot;Indexes should fit in RAM&quot; and similar variants</title>
                <link>https://jira.mongodb.org/browse/DOCS-14071</link>
                <project id="10380" key="DOCS">Documentation</project>
                    <description>&lt;h2&gt;&lt;a name=&quot;Description&quot;&gt;&lt;/a&gt;Description&lt;/h2&gt;

&lt;p&gt;We have numerous references in our documentation where we advise customers that &quot;indexes should fit in RAM&quot; for optimal performance.&lt;/p&gt;

&lt;p&gt;For example:&lt;br/&gt;
&lt;a href=&quot;https://docs.mongodb.com/manual/tutorial/ensure-indexes-fit-ram/&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://docs.mongodb.com/manual/tutorial/ensure-indexes-fit-ram/&lt;/a&gt;&lt;/p&gt;

&lt;p&gt;Notice we contradict the entire point of the page with the&lt;/p&gt;

&lt;p&gt;and&lt;/p&gt;

&lt;p&gt;&lt;a href=&quot;https://docs.mongodb.com/manual/applications/indexes/&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://docs.mongodb.com/manual/applications/indexes/&lt;/a&gt;&lt;br/&gt;
Where we say &quot;When your index fits in RAM, the system can avoid reading the index from disk and you get the fastest processing.&quot;&lt;/p&gt;

&lt;p&gt;There are a few problems with making these statements. While it&apos;s true that having indexes in RAM does make it so that you don&apos;t have to read them from disk THIS IS TRUE FOR ALL DATA USED BY THE APPLICATION. There&apos;s nothing special about having &quot;just&quot; indexes in RAM. The more of your &quot;entire database&quot; you have in RAM the better the performance will be.&lt;/p&gt;

&lt;p&gt;The working set is composed of both data and index pages and MongoDB offers no mechanism to pin specific data in cache. When a query executes it must sequentially read the relevant index keys by pulling their pages into cache and then pull the relevant document pages.&#160; This means that the working set needs to have both index pages and data pages for the most accessed data.&#160; We don&apos;t store an entire index in RAM... that would be a massive waste of RAM if only a small portion of the index is actually used for data access.&#160; &lt;/p&gt;

&lt;p&gt;This advice ignores how WiredTiger works and gives customers flawed guidance.&#160;When customers read these statements they inevitably open support cases where we need to walk them away from what our documentation says because the issue is, ultimately, more nuanced.&lt;/p&gt;

&lt;p&gt;An alternative approach would emphasize the need to manage the working set, describing how indexes and data access occurs in a running query, and how making queries efficient makes performance optimal. I&apos;m happy to assist if we&apos;d like to alter the approach we&apos;re taking now.&lt;/p&gt;
&lt;h2&gt;&lt;a name=&quot;Scopeofchanges&quot;&gt;&lt;/a&gt;Scope of changes&lt;/h2&gt;
&lt;h2&gt;&lt;a name=&quot;ImpacttoOtherDocs&quot;&gt;&lt;/a&gt;Impact to Other Docs&lt;/h2&gt;
&lt;h2&gt;&lt;a name=&quot;MVP%28WorkandDate%29&quot;&gt;&lt;/a&gt;MVP (Work and Date)&lt;/h2&gt;
&lt;h2&gt;&lt;a name=&quot;Resources%28ScopeorDesignDocs%2CInvision%2Cetc.%29&quot;&gt;&lt;/a&gt;Resources (Scope or Design Docs, Invision, etc.)&lt;/h2&gt;</description>
                <environment></environment>
        <key id="1576957">DOCS-14071</key>
            <summary>Remove all references to &quot;Indexes should fit in RAM&quot; and similar variants</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="10038" iconUrl="https://jira.mongodb.org/images/icons/subtask.gif" description="">Backlog</status>
                    <statusCategory id="2" key="new" colorName="default"/>
                                    <resolution id="-1">Unresolved</resolution>
                                        <assignee username="-1">Unassigned</assignee>
                                    <reporter username="shakir.sadikali@mongodb.com">Shakir Sadikali</reporter>
                        <labels>
                            <label>backlog</label>
                            <label>proactive</label>
                            <label>query</label>
                    </labels>
                <created>Wed, 23 Dec 2020 16:36:51 +0000</created>
                <updated>Mon, 22 Jan 2024 16:10:50 +0000</updated>
                                                                            <component>manual</component>
                    <component>Server</component>
                        <due>Fri, 25 Dec 2020 00:00:00 +0000</due>
                            <votes>7</votes>
                                    <watches>17</watches>
                                                                                                                <comments>
                            <comment id="3614902" author="asya" created="Fri, 12 Feb 2021 23:00:51 +0000"  >&lt;p&gt;I&#8217;m more concerned that we say full index should fit in RAM when right balanced indexes perform well with just the hot part of the index in RAM &lt;/p&gt;</comment>
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