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    <title>MongoDB Jira</title>
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    <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-9259] Don&apos;t move chunks for non-empty hashed collection during shardCollection cmd</title>
                <link>https://jira.mongodb.org/browse/SERVER-9259</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;div class=&quot;panel&quot; style=&quot;background-color: #EEEEEE;border-color: #ccc;border-width: 1px;&quot;&gt;&lt;div class=&quot;panelHeader&quot; style=&quot;border-bottom-width: 1px;border-bottom-color: #ccc;background-color: #6CB33F;&quot;&gt;&lt;b&gt;Issue Status as of March 27, 2014&lt;/b&gt;&lt;/div&gt;&lt;div class=&quot;panelContent&quot; style=&quot;background-color: #EEEEEE;&quot;&gt;
&lt;p&gt;&lt;b&gt;ISSUE SUMMARY&lt;/b&gt;&lt;br/&gt;
If a collection is being sharded with a hashed shard key, the initially created chunks are balanced across the shards immediately as part of the &lt;tt&gt;shardCollection&lt;/tt&gt; command (independent of the balancer). The intention was to only balance empty chunks, i.e. if the collection did not contain any documents yet. This bug moves the initial chunks even if the collection already contains documents. &lt;/p&gt;


&lt;p&gt;&lt;b&gt;USER IMPACT&lt;/b&gt;&lt;br/&gt;
The immediate balancing of initial chunks has the following undesireable effects: &lt;/p&gt;
&lt;ul&gt;
	&lt;li&gt;It unexpectedly moves potentially large amounts of data, which can have a noticeable impact on the cluster due to increased load on network and disks.&lt;/li&gt;
	&lt;li&gt;The initial chunks are moved whether the balancer is enabled or not, and regardless of a defined balancer window.&lt;/li&gt;
	&lt;li&gt;The &lt;tt&gt;shardCollection&lt;/tt&gt; command silently waits for the initial chunk moves to finish, and may appear to be stalled during that time.&lt;/li&gt;
&lt;/ul&gt;


&lt;p&gt;Forcefully aborting the &lt;tt&gt;shardCollection&lt;/tt&gt; command can leave the meta-data of the sharded collection in an undefined state. &lt;/p&gt;

&lt;p&gt;Increasing the number of initial chunks (with the &lt;tt&gt;numInitialChunks&lt;/tt&gt; option) causes more chunks to be moved and therefore aggravates the issue.&lt;/p&gt;

&lt;p&gt;&lt;b&gt;SOLUTION&lt;/b&gt;&lt;br/&gt;
The fix is to only trigger the initial chunk move logic if the collection is empty. &lt;/p&gt;

&lt;p&gt;&lt;b&gt;WORKAROUNDS&lt;/b&gt;&lt;br/&gt;
To work around the issue, only shard collections (on a hashed shard key) if they are empty. If you need to shard a non-empty collection, keep the number of initial chunks to the default, which is 2 for each shard. &lt;/p&gt;

&lt;p&gt;&lt;b&gt;PATCHES&lt;/b&gt;&lt;br/&gt;
The fix is included in the 2.4.10 production release and the 2.5.5 development release, which evolves into the 2.6.0 production release.&lt;/p&gt;
&lt;/div&gt;&lt;/div&gt;


&lt;h6&gt;&lt;a name=&quot;OriginalDescription&quot;&gt;&lt;/a&gt;Original Description&lt;/h6&gt;
&lt;p&gt;The comment in ShardCollectionCmd::run() says:&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;                // only initially move chunks when using a hashed shard key&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;                if (isHashedShardKey) {&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;&amp;nbsp;&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;                    // Reload the new config info.  If we created more than one initial chunk, then&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;                    // we need to move them around to balance.&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;                    ChunkManagerPtr chunkManager = config-&amp;gt;getChunkManager( ns , true );&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;                    ChunkMap chunkMap = chunkManager-&amp;gt;getChunkMap();&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;&amp;nbsp;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
			&lt;/tbody&gt;
&lt;/table&gt;
&lt;/div&gt;
&lt;p/&gt;

&lt;p&gt;But it&apos;s not checking that the collection is empty before moving the chunks directly from within the command.  This code block is only meant to be hit with empty collections in which case it is very fast and ensures even chunk distribution.&lt;/p&gt;</description>
                <environment></environment>
        <key id="70875">SERVER-9259</key>
            <summary>Don&apos;t move chunks for non-empty hashed collection during shardCollection cmd</summary>
                <type id="1" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14703&amp;avatarType=issuetype">Bug</type>
                                            <priority id="5" iconUrl="https://jira.mongodb.org/images/icons/priorities/trivial.svg">Trivial - P5</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="randolph@mongodb.com">Randolph Tan</assignee>
                                    <reporter username="randolph@mongodb.com">Randolph Tan</reporter>
                        <labels>
                    </labels>
                <created>Fri, 5 Apr 2013 14:55:21 +0000</created>
                <updated>Mon, 11 Jul 2016 17:57:04 +0000</updated>
                            <resolved>Wed, 12 Feb 2014 12:41:28 +0000</resolved>
                                    <version>2.4.1</version>
                                    <fixVersion>2.4.10</fixVersion>
                    <fixVersion>2.5.5</fixVersion>
                                    <component>Sharding</component>
                                        <votes>1</votes>
                                    <watches>7</watches>
                                                                                                                <comments>
                            <comment id="512122" author="xgen-internal-githook" created="Sun, 9 Mar 2014 15:28:03 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{u&apos;username&apos;: u&apos;renctan&apos;, u&apos;name&apos;: u&apos;Randolph Tan&apos;, u&apos;email&apos;: u&apos;randolph@10gen.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-9259&quot; title=&quot;Don&amp;#39;t move chunks for non-empty hashed collection during shardCollection cmd&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-9259&quot;&gt;&lt;del&gt;SERVER-9259&lt;/del&gt;&lt;/a&gt; Don&apos;t move chunks for non-empty hashed collection during shardCollection cmd&lt;br/&gt;
Branch: v2.4&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/fcabc21a3e9336869af2598aaa7ad86012d467cb&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/fcabc21a3e9336869af2598aaa7ad86012d467cb&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="488401" author="xgen-internal-githook" created="Mon, 27 Jan 2014 21:21:21 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{u&apos;username&apos;: u&apos;renctan&apos;, u&apos;name&apos;: u&apos;Randolph Tan&apos;, u&apos;email&apos;: u&apos;randolph@10gen.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-9259&quot; title=&quot;Don&amp;#39;t move chunks for non-empty hashed collection during shardCollection cmd&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-9259&quot;&gt;&lt;del&gt;SERVER-9259&lt;/del&gt;&lt;/a&gt; Don&apos;t move chunks for non-empty hashed collection during shardCollection cmd&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/e9423c914367cf0ae89d27999e824da70864c59c&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/e9423c914367cf0ae89d27999e824da70864c59c&lt;/a&gt;&lt;/p&gt;</comment>
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