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        <version>9.7.1</version>
        <build-number>970001</build-number>
        <build-date>13-04-2023</build-date>
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<item>
            <title>[SERVER-66] do memcmp before memcpy for update?</title>
                <link>https://jira.mongodb.org/browse/SERVER-66</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;should we compare the old and new objects an update before writing?&lt;/p&gt;

&lt;p&gt;what i&apos;m thinking is that if we&apos;re writing the same object out - it may incur a dirty bit set on the page when it doesn&apos;t need to be.&lt;br/&gt;
which could lead to more disk activity.&lt;/p&gt;

&lt;p&gt;we&apos;ll already have to read the page in - so its just the comparison that we add.  and we only have to do it if they are exactly the same size.  my guess is if the object changes - most of the time the size will change anyway.  &lt;/p&gt;</description>
                <environment></environment>
        <key id="10158">SERVER-66</key>
            <summary>do memcmp before memcpy for update?</summary>
                <type id="4" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14710&amp;avatarType=issuetype">Improvement</type>
                                            <priority id="4" iconUrl="https://jira.mongodb.org/images/icons/priorities/minor.svg">Minor - P4</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="dwight@mongodb.com">Dwight Merriman</assignee>
                                    <reporter username="eliot">Eliot Horowitz</reporter>
                        <labels>
                    </labels>
                <created>Thu, 21 May 2009 23:48:03 +0000</created>
                <updated>Mon, 19 May 2014 22:09:04 +0000</updated>
                            <resolved>Fri, 22 May 2009 10:13:46 +0000</resolved>
                                                                                        <votes>0</votes>
                                    <watches>1</watches>
                                                                                                                <comments>
                            <comment id="431287" author="jblackburn" created="Wed, 25 Sep 2013 17:53:08 +0000"  >&lt;p&gt;This would fix a major performance issue we&apos;re seeing.&lt;/p&gt;

&lt;p&gt;We have a schema, much like gridfs, the main differenced is that chunks are sha&apos;d and shared by multiple versions of a &apos;file&apos;.  The chunks have an indexed parent array field, so fetching data is fast, and data is de-deduped on insert.&lt;/p&gt;

&lt;p&gt;When it comes to deleting versions of a &apos;file&apos;, however we see huge I/O load as all the chunk data is re-flushed to disk.  We could construct our schema such that the only the last field of the chunk doc changed - i.e. the parents field.  It would be nice if only the few pages that need to be changed are written back as this can effectively DOS our Mongo cluster.&lt;/p&gt;


</comment>
                            <comment id="10138" author="dwight_10gen" created="Fri, 22 May 2009 10:13:46 +0000"  >&lt;p&gt;madness &lt;br/&gt;
&lt;img class=&quot;emoticon&quot; src=&quot;https://jira.mongodb.org/images/icons/emoticons/smile.png&quot; height=&quot;16&quot; width=&quot;16&quot; align=&quot;absmiddle&quot; alt=&quot;&quot; border=&quot;0&quot;/&gt;&lt;/p&gt;</comment>
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