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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-55451] Create (or reuse) the named FIFO proxy to simulate mongod process entering uninterruptible sleep state</title>
                <link>https://jira.mongodb.org/browse/SERVER-55451</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;h2&gt;&lt;a name=&quot;Background&quot;&gt;&lt;/a&gt;Background&lt;/h2&gt;

&lt;p&gt;In HELP ticket I suspect the scenario when a faulty disk made all disk I/O to be blocked for indefinite time, which caused the process to enter the uninterruptible sleep state. The main culprit of this state is that when SIGKILL is issued the process is not killed because it&apos;s blocked on a syscall. The user killed the primary mongod server with -9 but it was not killed. After 13 minutes after SIGKILL, the user had to shut down the Amazon EC2 instance to break down hung sessions from multiple mongos proxies to the faulty primary.&lt;/p&gt;

&lt;p&gt;More background on why `kill -9` will never kill the process in the uninterruptible sleep state:&lt;br/&gt;
&lt;a href=&quot;https://askubuntu.com/questions/59811/kill-pid-not-really-killing-the-process-why&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://askubuntu.com/questions/59811/kill-pid-not-really-killing-the-process-why&lt;/a&gt;&lt;/p&gt;

&lt;p&gt;Various tricks people use to simulate the uninterruptible sleep state:&lt;br/&gt;
&lt;a href=&quot;https://unix.stackexchange.com/questions/134888/simulate-an-unkillable-process-in-d-state&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://unix.stackexchange.com/questions/134888/simulate-an-unkillable-process-in-d-state&lt;/a&gt;&lt;/p&gt;

&lt;p&gt;More background on why kernel prevents killing process in this kind of state:&lt;br/&gt;
&lt;a href=&quot;https://stackoverflow.com/questions/223644/what-is-an-uninterruptible-process&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://stackoverflow.com/questions/223644/what-is-an-uninterruptible-process&lt;/a&gt;&lt;/p&gt;

&lt;p&gt;and LWN article: &lt;a href=&quot;https://lwn.net/Articles/288056/&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://lwn.net/Articles/288056/&lt;/a&gt;&lt;/p&gt;

&lt;h2&gt;&lt;a name=&quot;Proposal&quot;&gt;&lt;/a&gt;Proposal&lt;/h2&gt;

&lt;p&gt;The trick we can use is the idea that a named FIFO pipe is also a blocked syscall that will make the process uninterruptible if there is no data for some time. Two changes are needed:&lt;/p&gt;

&lt;p&gt;1. We need to either write or reuse a proxy server between FIFO and hard disk. It will present itself as multiple named pipes, and redirect each pipe to a file on hard disk.&lt;/p&gt;

&lt;p&gt;2. Modify mongod / WT to detect a special file format and ping the FIFO proxy with special command instead of creating a new file itself. The proxy should listen for those requests and create a new FIFO when asked. Then mongod can open the FIFO like a regular file, the rest of the code is unchanged.&lt;/p&gt;

&lt;p&gt;To simulate the outage, the FIFO proxy should be instructed to stop replying to R/W requests. &lt;/p&gt;

&lt;h3&gt;&lt;a name=&quot;Notthesameasnetworkproxy&quot;&gt;&lt;/a&gt;Not the same as network proxy&lt;/h3&gt;

&lt;p&gt;Please note that we already have mongobridge to simulate network errors, however this is not the same. The mongo bridge cannot make the outage in the mongod, it can only make the client to think that mongod has an outage, which is very different from the scenario in HELP ticket.&lt;/p&gt;</description>
                <environment></environment>
        <key id="1656551">SERVER-55451</key>
            <summary>Create (or reuse) the named FIFO proxy to simulate mongod process entering uninterruptible sleep state</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="backlog-server-storage-engines">Backlog - Storage Engines Team</assignee>
                                    <reporter username="andrew.shuvalov@mongodb.com">Andrew Shuvalov</reporter>
                        <labels>
                    </labels>
                <created>Tue, 23 Mar 2021 17:20:01 +0000</created>
                <updated>Tue, 6 Dec 2022 01:29:35 +0000</updated>
                            <resolved>Wed, 24 Mar 2021 15:18:32 +0000</resolved>
                                                                                        <votes>0</votes>
                                    <watches>4</watches>
                                                                                                                <comments>
                            <comment id="3681626" author="JIRAUSER1256988" created="Wed, 24 Mar 2021 15:18:32 +0000"  >&lt;p&gt;Preempted by a better idea in &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-55486&quot; title=&quot;Design new integration test facility to use Device-mapper to reproduce disk outage&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-55486&quot;&gt;&lt;del&gt;SERVER-55486&lt;/del&gt;&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="3680044" author="JIRAUSER1256988" created="Tue, 23 Mar 2021 18:59:54 +0000"  >&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=aaron.redalen&quot; class=&quot;user-hover&quot; rel=&quot;aaron.redalen&quot;&gt;aaron.redalen&lt;/a&gt; suggested to use dm-delay &lt;a href=&quot;https://www.kernel.org/doc/html/latest/admin-guide/device-mapper/delay.html&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://www.kernel.org/doc/html/latest/admin-guide/device-mapper/delay.html&lt;/a&gt; as an alternative to my approach. Please give it a second thought as much simpler approach to simulate disk failure, and if you think it will work compatible with the way we use storage.&lt;/p&gt;</comment>
                            <comment id="3679882" author="bruce.lucas@10gen.com" created="Tue, 23 Mar 2021 17:52:45 +0000"  >&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=andrew.shuvalov&quot; class=&quot;user-hover&quot; rel=&quot;andrew.shuvalov&quot;&gt;andrew.shuvalov&lt;/a&gt;&#160;if the goal is just to make mongod uninterruptible by kill -9 and if some thread waiting on a fifo with no data is sufficient to do that, why does that need to be in the storage engine?&lt;/p&gt;</comment>
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