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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-42022] Attempt to remove initial sync missing document fetching</title>
                <link>https://jira.mongodb.org/browse/SERVER-42022</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;We currently think this was only necessary for mmapv1 when updates could cause a collection scan to miss a document. If we could remove this step, it could significantly speed up initial sync in certain workloads and simplify initial sync.&lt;/p&gt;

&lt;p&gt;This would require the storage engine API to prohibit updates from causing collection scans to miss a document.&lt;/p&gt;</description>
                <environment></environment>
        <key id="829115">SERVER-42022</key>
            <summary>Attempt to remove initial sync missing document fetching</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="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="13201">Fixed</resolution>
                                        <assignee username="jesse@mongodb.com">A. Jesse Jiryu Davis</assignee>
                                    <reporter username="judah.schvimer@mongodb.com">Judah Schvimer</reporter>
                        <labels>
                    </labels>
                <created>Mon, 1 Jul 2019 13:33:31 +0000</created>
                <updated>Sun, 29 Oct 2023 22:19:24 +0000</updated>
                            <resolved>Wed, 21 Aug 2019 16:15:54 +0000</resolved>
                                                    <fixVersion>4.3.1</fixVersion>
                                    <component>Replication</component>
                                        <votes>0</votes>
                                    <watches>13</watches>
                                                                                                                <comments>
                            <comment id="2398530" author="jesse" created="Wed, 28 Aug 2019 21:57:00 +0000"  >&lt;p&gt;Not backporting: this change is permissible in 4.4 because the sync source won&apos;t be MMAPv1, but not permissible in 4.2, which could sync from a 4.0 mongod running MMAPv1.&lt;/p&gt;</comment>
                            <comment id="2381121" author="xgen-internal-githook" created="Wed, 21 Aug 2019 15:55:32 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;username&apos;: &apos;ajdavis&apos;, &apos;email&apos;: &apos;jesse@mongodb.com&apos;, &apos;name&apos;: &apos;A. Jesse Jiryu Davis&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-42022&quot; title=&quot;Attempt to remove initial sync missing document fetching&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-42022&quot;&gt;&lt;del&gt;SERVER-42022&lt;/del&gt;&lt;/a&gt; Remove missing-document fetcher&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/609e2697c2619925073d4ac0f57f33e789d05100&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/609e2697c2619925073d4ac0f57f33e789d05100&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="2365604" author="david.golden" created="Fri, 9 Aug 2019 23:36:46 +0000"  >&lt;p&gt;Please also recheck vs&#160;&lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-38860&quot; title=&quot;Positional array update behavior of applyOps on invalid field varies on different versions&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-38860&quot;&gt;&lt;del&gt;SERVER-38860&lt;/del&gt;&lt;/a&gt;, which has this line:&lt;/p&gt;
&lt;blockquote&gt;&lt;p&gt;there is no idempotency concern because if an error is returned, initial sync will refetch the document as if the document is missing&lt;/p&gt;&lt;/blockquote&gt;</comment>
                            <comment id="2315656" author="william.schultz" created="Mon, 8 Jul 2019 17:56:11 +0000"  >&lt;p&gt;Perhaps verifying the correctness of this change would be suitable for a small scale formal model. It could be another good candidate for trying out TLA+ and its model checker. I know that &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=max.hirschhorn&quot; class=&quot;user-hover&quot; rel=&quot;max.hirschhorn&quot;&gt;max.hirschhorn&lt;/a&gt; and &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=siyuan.zhou&quot; class=&quot;user-hover&quot; rel=&quot;siyuan.zhou&quot;&gt;siyuan.zhou&lt;/a&gt; have previously explored similar topics e.g. PM-825.&lt;/p&gt;</comment>
                    </comments>
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                                                                <inwardlinks description="is depended on by">
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            <issuekey id="326892">SERVER-26780</issuekey>
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                            <issuelinktype id="10320">
                    <name>Documented</name>
                                                                <inwardlinks description="is documented by">
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            <issuekey id="902802">DOCS-12975</issuekey>
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                            <issuelinktype id="10012">
                    <name>Related</name>
                                            <outwardlinks description="related to">
                                        <issuelink>
            <issuekey id="160393">SERVER-15410</issuekey>
        </issuelink>
                            </outwardlinks>
                                                                <inwardlinks description="is related to">
                                        <issuelink>
            <issuekey id="235577">SERVER-21057</issuekey>
        </issuelink>
                            </inwardlinks>
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                            <customfieldvalue>During initial sync, a replica clones all collections, while at the same time buffer its sync source&amp;#39;s new oplog entries. Once cloning completes, the replica sets its &amp;quot;minValid&amp;quot; timestamp to the latest oplog entry, and replays the oplog up to minValid. At this point the replica has constructed a consistent snapshot of the data, and it can transition to a secondary.&lt;br/&gt;
&lt;br/&gt;
In MMAPv1 days, it was possible to miss a document while cloning a collection, even if the document existed during the whole time we were cloning, because the document was moved during the clone. Therefore, during recovery or initial sync, when a replica would apply an update operation that referred to a missing document (either from a regular update or the &amp;quot;applyOps&amp;quot; command) it would attempt to fetch the document from the sync source, in case the document had been missed during cloning. If the document was found, the replica would advance its minValid timestamp to the sync source&amp;#39;s current opTime, setting back its progress toward achieving a consistent snapshot.&lt;br/&gt;
&lt;br/&gt;
In MongoDB 4.3 we no longer support MMAPv1, and we don&amp;#39;t even support syncing from a server that uses MMAPv1. Post-MMAPv1 we can&amp;#39;t miss documents during collection clone, so it is no longer necessary to fetch missing documents, nor to advance minValid. If an update refers to a missing document, then there must be a later oplog entry that deletes the document, so the replica can skip the update operation.&lt;br/&gt;
&lt;br/&gt;
We don&amp;#39;t have much documentation of this process, so not much has to change. replSetGetStatus.initialSyncStatus.fetchedMissingDocs field is removed, see two references in replSetGetStatus.txt and one in getLog.txt. There may be other places that need updating that I have no found, however!</customfieldvalue>

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