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    <description>This file is an XML representation of an issue</description>
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        <build-number>970001</build-number>
        <build-date>13-04-2023</build-date>
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<item>
            <title>[SERVER-4495] keep an extent&apos;s linked list of records ordered by disk loc</title>
                <link>https://jira.mongodb.org/browse/SERVER-4495</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;I wrote this up in June 2009, so might be out of date.  Just putting it in jira for safekeeping.  Just looking at what I wrote back then, I think I preferred the deleted record globbing approach because it didn&apos;t necessitate a data format change.  But there could be a minor (backwards compatible ?) format change that we could do in addition to the globbing, would be more robust and allow us to do some additional validation.&lt;/p&gt;


&lt;p&gt;Right now records and deleted records are formatted so we can scan&lt;br/&gt;
through them using linked lists, but I believe the data format is such&lt;br/&gt;
that we can alternatively scan along an extent by looking at whatever&lt;br/&gt;
record or deleted record comes right after a given record or deleted&lt;br/&gt;
record on disk. (If I know the offset of a given record, I can just&lt;br/&gt;
add the length of that record to its offset to get the offset of the&lt;br/&gt;
next record / deleted record.) So in this way if you have a deleted&lt;br/&gt;
record you want to reclaim, you can just scan along the disk for the&lt;br/&gt;
next regular record. Since regular records are in a doubly linked&lt;br/&gt;
list, you can then splice in the deleted record as a new record.&lt;/p&gt;

&lt;p&gt;One issue is that it may not be possible to differentiate a deleted&lt;br/&gt;
record from a regular record just by looking at the bit&lt;br/&gt;
representation. A couple ways of dealing with this. One, change the&lt;br/&gt;
data format. I think a nicer way would be to get aggressive about&lt;br/&gt;
aggregating deleted records when possible. By this I mean when we&lt;br/&gt;
process a write operation we never allow two deleted records to be&lt;br/&gt;
next to each other as a result of that operation; the deleted records&lt;br/&gt;
must be merged into one. If we adopt this policy, then the region on&lt;br/&gt;
disk directly following a deleted record must be a real record, so we&lt;br/&gt;
can easily splice a deleted record into the regular record list. (We&lt;br/&gt;
already track the last record per extent in the extent header, so if a&lt;br/&gt;
deleted record at the end of an extent is to be reclaimed we can still&lt;br/&gt;
splice it in easily.)&lt;/p&gt;

&lt;p&gt;If I remember correctly we do some deleted record globbing like this&lt;br/&gt;
already, but not after every write operation. If records are ordered,&lt;br/&gt;
we can do globbing efficiently on every write: if a record is to be&lt;br/&gt;
converted to a deleted record, we find the regular records adjacent to&lt;br/&gt;
the regular record to be deleted and then convert all disk space&lt;br/&gt;
between those to a single deleted record (the 1 regular record and &amp;lt;=2&lt;br/&gt;
deleted records in this region can be easily unhooked from their&lt;br/&gt;
doubly linked lists).&lt;/p&gt;</description>
                <environment></environment>
        <key id="26638">SERVER-4495</key>
            <summary>keep an extent&apos;s linked list of records ordered by disk loc</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="2">Won&apos;t Fix</resolution>
                                        <assignee username="-1">Unassigned</assignee>
                                    <reporter username="aaron">Aaron Staple</reporter>
                        <labels>
                    </labels>
                <created>Thu, 15 Dec 2011 01:51:59 +0000</created>
                <updated>Mon, 19 Dec 2011 07:16:05 +0000</updated>
                            <resolved>Fri, 16 Dec 2011 06:45:27 +0000</resolved>
                                                                                        <votes>0</votes>
                                    <watches>2</watches>
                                                                                                                <comments>
                            <comment id="73820" author="eliot" created="Fri, 16 Dec 2011 06:45:27 +0000"  >&lt;p&gt;There are other things we&apos;re planning on doing with the free list which make this less useful.&lt;/p&gt;</comment>
                            <comment id="73235" author="aaron" created="Thu, 15 Dec 2011 02:06:12 +0000"  >&lt;p&gt;Hmm, updating the deleted record lists may not be as easy as I wrote up there.&lt;/p&gt;</comment>
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