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    <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-79329] Exclude lagged secondaries when selecting hosts for mirrored reads</title>
                <link>https://jira.mongodb.org/browse/SERVER-79329</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;The host selection mechanism of mirrored read (which selects &lt;a href=&quot;https://github.com/mongodb/mongo/blob/2ed3c403291e3b761a089545e39205756b2bb40c/src/mongo/db/mirroring_sampler.cpp#L101&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;all electable secondaries&lt;/a&gt;) can be improved to exclude secondaries that are lagged behind for certain amount of time. This is because&lt;/p&gt;
&lt;ol&gt;
	&lt;li&gt;Lagged secondaries are pretty unlikely to be elected as primary.&lt;/li&gt;
	&lt;li&gt;Due to locality, queries on primary are more likely targeting latest writes, and since lagged secondaries may not have the latest inserted documents, the mirrored reads may end up reading nothing, and is simply wasted. And I guess even if it finds the document, it may result in churns in the cache.&lt;/li&gt;
	&lt;li&gt;It can help void extra load as lagged secondaries trying to catch up.&lt;/li&gt;
&lt;/ol&gt;
</description>
                <environment></environment>
        <key id="2400526">SERVER-79329</key>
            <summary>Exclude lagged secondaries when selecting hosts for mirrored reads</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="backlog-server-servicearch">Backlog - Service Architecture</assignee>
                                    <reporter username="wenbin.zhu@mongodb.com">Wenbin Zhu</reporter>
                        <labels>
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                <created>Tue, 25 Jul 2023 18:43:47 +0000</created>
                <updated>Tue, 1 Aug 2023 20:26:19 +0000</updated>
                                                                                                <votes>0</votes>
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