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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-14003] 2.6 mongos does not support overlapped writes at w:0</title>
                <link>https://jira.mongodb.org/browse/SERVER-14003</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;Shows up as a performance degradation for applications that use w:0 and getLastError in a sharded environment when going from 2.4 to 2.6:&lt;/p&gt;

&lt;p&gt;To reproduce: create a collection, mongodump, mongorestore via mongos to single-shard cluster (single mongod, single config server). Doesn&apos;t seem to matter if collection is sharded or not.&lt;/p&gt;

&lt;p&gt;Here are some numbers for one data set:&lt;/p&gt;
&lt;p/&gt;
&lt;div id=&quot;syntaxplugin&quot; class=&quot;syntaxplugin&quot; style=&quot;border: 1px dashed #bbb; border-radius: 5px !important; overflow: auto; max-height: 30em;&quot;&gt;
&lt;table cellspacing=&quot;0&quot; cellpadding=&quot;0&quot; border=&quot;0&quot; width=&quot;100%&quot; style=&quot;font-size: 1em; line-height: 1.4em !important; font-weight: normal; font-style: normal; color: black;&quot;&gt;
		&lt;tbody &gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;  margin-top: 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;2.4.10, standalone                      26s&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&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;2.6.1, standalone                       26s&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&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;2.4.10, 1x shard, 1x mongos, sharded    58s&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&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;2.6.1, 1x shard, 1x mongos, sharded    352s&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
			&lt;/tbody&gt;
&lt;/table&gt;
&lt;/div&gt;
&lt;p/&gt;

&lt;p&gt;Another data set, showing that sharded or unsharded shows same degradation:&lt;/p&gt;
&lt;p/&gt;
&lt;div id=&quot;syntaxplugin&quot; class=&quot;syntaxplugin&quot; style=&quot;border: 1px dashed #bbb; border-radius: 5px !important; overflow: auto; max-height: 30em;&quot;&gt;
&lt;table cellspacing=&quot;0&quot; cellpadding=&quot;0&quot; border=&quot;0&quot; width=&quot;100%&quot; style=&quot;font-size: 1em; line-height: 1.4em !important; font-weight: normal; font-style: normal; color: black;&quot;&gt;
		&lt;tbody &gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;  margin-top: 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;2.4.10, standalone                       5s&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&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;2.6.1, standalone                        5s&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&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;2.4.10, 1x shard, 1x mongos, unsharded   5s&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&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;2.6.1, 1x shard, 1x mongos, unsharded   45s&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&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;2.4.10, 1x shard, 1x mongos, sharded     8s&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&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;2.6.1, 1x shard, 1x mongos, sharded     52s&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
			&lt;/tbody&gt;
&lt;/table&gt;
&lt;/div&gt;
&lt;p/&gt;</description>
                <environment></environment>
        <key id="137365">SERVER-14003</key>
            <summary>2.6 mongos does not support overlapped writes at w:0</summary>
                <type id="2" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14711&amp;avatarType=issuetype">New Feature</type>
                                            <priority id="2" iconUrl="https://jira.mongodb.org/images/icons/priorities/critical.svg">Critical - P2</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="2">Won&apos;t Fix</resolution>
                                        <assignee username="backlog-server-sharding">[DO NOT USE] Backlog - Sharding Team</assignee>
                                    <reporter username="bruce.lucas@mongodb.com">Bruce Lucas</reporter>
                        <labels>
                    </labels>
                <created>Tue, 20 May 2014 19:14:06 +0000</created>
                <updated>Tue, 6 Dec 2022 05:05:40 +0000</updated>
                            <resolved>Tue, 24 Apr 2018 15:22:53 +0000</resolved>
                                    <version>2.6.1</version>
                                                    <component>Sharding</component>
                                        <votes>4</votes>
                                    <watches>25</watches>
                                                                                                                <comments>
                            <comment id="1872678" author="greg.mckeon" created="Tue, 24 Apr 2018 15:22:53 +0000"  >&lt;p&gt;Both w:0 writes and gle are soon to be deprecated and 2.6 is no longer supported, so we&apos;re closing this as won&apos;t fix.&lt;/p&gt;</comment>
                            <comment id="1043653" author="schwerin" created="Fri, 25 Sep 2015 17:28:39 +0000"  >&lt;p&gt;The purpose of getLastError is to allow a client application to learn about the success or failure of the immediately prior unconfirmed write on the connection, and to wait for a write concern to be satisfied on that write. As such, mongos must track information on all unconfirmed writes to satisfy getLastError requests that may come in the future. There are two ways to do this. One is the way that we do in 2.6 and beyond, which is to implement unconfirmed writes to mongos as confirmed writes to mongod, and store relevant return data. The other way, which we did in 2.4 and before, was to tightly bind client connections with corresponding shard connections in mongos. That is, if  a client contacts mongos on mongos connection 1 and does an unconfirmed write, and mongos takes shard1 connection 2 out of the pool to perform that write on the shard, it must not re-use shard 1 connection 2 for any purpose other than to satisfy connection 1&apos;s getLastError request until that request arrives or the client closes connection 1.  This leads to &lt;em&gt;very&lt;/em&gt; high connection counts and very complicated accounting in mongos. That complicated accounting also prevents other necessary enhancements to sharding, so we removed it.&lt;/p&gt;</comment>
                            <comment id="653235" author="greg_10gen" created="Wed, 9 Jul 2014 16:25:15 +0000"  >&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=gianfranco&quot; class=&quot;user-hover&quot; rel=&quot;gianfranco&quot;&gt;gianfranco&lt;/a&gt; Keeping this open as the more general issue - at some point we&apos;d like to address.&lt;/p&gt;</comment>
                            <comment id="607429" author="gianfranco" created="Wed, 4 Jun 2014 09:00:26 +0000"  >&lt;p&gt;I think this should be closed as duplicate of &lt;a href=&quot;https://jira.mongodb.org/browse/TOOLS-159&quot; title=&quot;Tools should use (batch) write commands&quot; class=&quot;issue-link&quot; data-issue-key=&quot;TOOLS-159&quot;&gt;&lt;del&gt;SERVER-12016&lt;/del&gt;&lt;/a&gt;. As that&apos;s the proper fix.&lt;/p&gt;</comment>
                            <comment id="590638" author="greg_10gen" created="Tue, 20 May 2014 19:30:16 +0000"  >&lt;p&gt;This is known behavior - it&apos;s due to the fact that the mongo tools do not use batch operations, and mongos converts old-style w:0 writes into w:1 writes.&lt;/p&gt;</comment>
                    </comments>
                <issuelinks>
                            <issuelinktype id="10010">
                    <name>Duplicate</name>
                                                                <inwardlinks description="is duplicated by">
                                        <issuelink>
            <issuekey id="259985">SERVER-22260</issuekey>
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                            <issuelinktype id="10012">
                    <name>Related</name>
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                                        <issuelink>
            <issuekey id="270995">PYTHON-1069</issuekey>
        </issuelink>
                            </outwardlinks>
                                                                <inwardlinks description="is related to">
                                        <issuelink>
            <issuekey id="137653">DOCS-3456</issuekey>
        </issuelink>
            <issuelink>
            <issuekey id="101681">TOOLS-159</issuekey>
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