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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-27597] The migration critical section does not need to stop reads</title>
                <link>https://jira.mongodb.org/browse/SERVER-27597</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;Currently when a shard is in the migration critical section it will block both read and write operation. The writes certainly need to be blocked to allow the last bits of the migration to be transferred, but there is no real need to block the reads.&lt;/p&gt;

&lt;p&gt;We could use the mode of the global lock to differentiate reads from writes.&lt;/p&gt;

&lt;p&gt;With this change the critical section stall would be only as long as the write on the config server is (instead of also including transferring the latest changes from donor to recipient).&lt;/p&gt;

&lt;p&gt;One thing which this may exacerbate is the length of the critical section if a large number of concurrent reads thrash the cache and cause the final batch of documents to take longer time to transfer.&lt;/p&gt;</description>
                <environment></environment>
        <key id="343269">SERVER-27597</key>
            <summary>The migration critical section does not need to stop reads</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="kaloian.manassiev@mongodb.com">Kaloian Manassiev</assignee>
                                    <reporter username="kaloian.manassiev@mongodb.com">Kaloian Manassiev</reporter>
                        <labels>
                    </labels>
                <created>Fri, 6 Jan 2017 16:29:35 +0000</created>
                <updated>Thu, 24 Jan 2019 06:30:44 +0000</updated>
                            <resolved>Tue, 13 Jun 2017 20:24:46 +0000</resolved>
                                    <version>3.0.15</version>
                    <version>3.2.13</version>
                    <version>3.4.4</version>
                    <version>3.5.1</version>
                                    <fixVersion>3.5.9</fixVersion>
                                    <component>Sharding</component>
                                        <votes>1</votes>
                                    <watches>13</watches>
                                                                                                                <comments>
                            <comment id="2123845" author="wangke" created="Wed, 23 Jan 2019 08:54:11 +0000"  >&lt;p&gt;hi, it seems the fix only make donor permit read operation in step 5. It has little improvement to our mongo sharding cluster(v3.4.18 with the patch) since step 5 take 10ms while step 6 may take 200-300 ms to commit and refresh catalogCache.&lt;/p&gt;

&lt;p&gt;What if read operations are not blocked in step 6?&lt;/p&gt;</comment>
                            <comment id="1597074" author="xgen-internal-githook" created="Wed, 14 Jun 2017 19:16:44 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{u&apos;username&apos;: u&apos;kaloianm&apos;, u&apos;name&apos;: u&apos;Kaloian Manassiev&apos;, u&apos;email&apos;: u&apos;kaloian.manassiev@mongodb.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-27597&quot; title=&quot;The migration critical section does not need to stop reads&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-27597&quot;&gt;&lt;del&gt;SERVER-27597&lt;/del&gt;&lt;/a&gt; Disable migration_critical_section_concurrency.js from the mixed shard version suite&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/629985c5ffacf32f71b17a0e797fcd74cf7b1f9d&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/629985c5ffacf32f71b17a0e797fcd74cf7b1f9d&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="1596012" author="xgen-internal-githook" created="Tue, 13 Jun 2017 20:23:51 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{u&apos;username&apos;: u&apos;kaloianm&apos;, u&apos;name&apos;: u&apos;Kaloian Manassiev&apos;, u&apos;email&apos;: u&apos;kaloian.manassiev@mongodb.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-27597&quot; title=&quot;The migration critical section does not need to stop reads&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-27597&quot;&gt;&lt;del&gt;SERVER-27597&lt;/del&gt;&lt;/a&gt; Delay blocking read operations during the migration critical section&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/044420833af14c0ad011d69288c448a0dd95bb22&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/044420833af14c0ad011d69288c448a0dd95bb22&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="1566047" author="schwerin" created="Fri, 5 May 2017 20:33:48 +0000"  >&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=renctan&quot; class=&quot;user-hover&quot; rel=&quot;renctan&quot;&gt;renctan&lt;/a&gt;, you are correct that it can make the range deleter wait on more queries, but the critical section is going to end eventually, at which point new queries for the old ranges will stop arriving. I think shifting waiting onto the range deleter is a pretty good tradeoff.&lt;/p&gt;</comment>
                            <comment id="1471389" author="renctan" created="Fri, 6 Jan 2017 18:42:10 +0000"  >&lt;p&gt;However, this can have a side effect of adding more queries the range deleter has to wait on.&lt;/p&gt;</comment>
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