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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>
    </build-info>


<item>
            <title>[SERVER-46435] Alternatives to using the lookaside-score for snapshot (history) window control [v4.2 only]</title>
                <link>https://jira.mongodb.org/browse/SERVER-46435</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;A few potential solutions I can think of:&lt;br/&gt;
1) Consider (dirty-bytes-in-cache / total-bytes-in-cache == 0.20 || bytes-in-use-in-cache / total-bytes-in-cache == 0.95) as cache pressure and reduce window size.&lt;br/&gt;
2) Make the window size constant and only user adjustable.&lt;br/&gt;
3) Make the window have a settable minimum value (new startup+runtime server parameter)&lt;/p&gt;</description>
                <environment></environment>
        <key id="1192825">SERVER-46435</key>
            <summary>Alternatives to using the lookaside-score for snapshot (history) window control [v4.2 only]</summary>
                <type id="3" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14718&amp;avatarType=issuetype">Task</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="12300">Won&apos;t Do</resolution>
                                        <assignee username="backlog-server-execution">Backlog - Storage Execution Team</assignee>
                                    <reporter username="dianna.hohensee@mongodb.com">Dianna Hohensee</reporter>
                        <labels>
                    </labels>
                <created>Wed, 26 Feb 2020 21:06:02 +0000</created>
                <updated>Tue, 6 Dec 2022 02:34:43 +0000</updated>
                            <resolved>Mon, 1 Feb 2021 19:43:26 +0000</resolved>
                                                                    <component>Storage</component>
                                        <votes>0</votes>
                                    <watches>3</watches>
                                                                                                                <comments>
                            <comment id="3593300" author="connie.chen" created="Mon, 1 Feb 2021 19:43:27 +0000"  >&lt;p&gt;We would be potentially destabilizing 4.2 with this change, closing as won&apos;t do.&#160;&lt;/p&gt;</comment>
                            <comment id="3072727" author="dianna.hohensee" created="Wed, 6 May 2020 20:40:32 +0000"  >&lt;p&gt;The WT eviction_trigger configuration defaults to 95% and is when application threads start to perform eviction. At that point, performance is starting to suffer / become unreliable.&lt;/p&gt;

&lt;p&gt;WT also has an eviction_target configuration that defaults to 80% at which it tries to keep the cache fullness. WT eviction threads are active when this threshold is reached. So WT seems to try to keep the cache relatively full, so that reads are fast, I imagine.&lt;/p&gt;

&lt;p&gt;WT also has an eviction_dirty_target (default 5%) and eviction_dirty_trigger (default 20%). These behave similarly and can co-opt application threads for eviction.&lt;/p&gt;

&lt;p&gt;I&apos;ve been looking at the information here &lt;a href=&quot;http://source.wiredtiger.com/3.2.1/tune_cache.html&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;http://source.wiredtiger.com/3.2.1/tune_cache.html&lt;/a&gt; to conclude the above.&lt;/p&gt;

&lt;p&gt;-------------------------------------------------------------------------------&lt;/p&gt;

&lt;p&gt;Two potential solutions I can think of:&lt;br/&gt;
1) Consider (dirty-bytes-in-cache / total-bytes-in-cache == 0.20 || bytes-in-use-in-cache / total-bytes-in-cache == 0.95) as cache pressure and reduce window size.&lt;br/&gt;
2) Make the window size constant and only user adjustable.&lt;/p&gt;</comment>
                            <comment id="3031939" author="dianna.hohensee" created="Wed, 8 Apr 2020 20:32:21 +0000"  >&lt;p&gt;Lookaside table inserts are not logically going to work because insert frequency could drop while cache pressure remains high. And it is thought (unconfirmed) that inserts can occur before there&apos;s any significant cache pressure.&lt;/p&gt;

&lt;p&gt;Luke Pearson suggested these metrics:&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;Cache bytes max: Total configured cache size&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;Cache bytes in use: total amount of bytes in use&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;Cache bytes dirty: Amount of dirty bytes in cache (internal + leaf pages).&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;Cache bytes internal: Bytes belonging to internal pages in the cache.&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;Cache bytes leaf: Bytes belonging to leaf pages in the cache.&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;   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;Conn_cache.c&#160;__wt_cache_stats_update&#160;is a good place to look at where the values are being set.&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;So I&apos;ll look into them.&lt;/p&gt;</comment>
                            <comment id="2915090" author="connie.chen" created="Thu, 27 Feb 2020 20:39:19 +0000"  >&lt;p&gt;Ensure to confirm with WT that this is change is correct&lt;/p&gt;</comment>
                    </comments>
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