Date: Wed, 16 Mar 2022 13:55:58 -0400 From: Karl Denninger <karl@denninger.net> To: freebsd-fs@freebsd.org Subject: md option "async" risks kernel deadlock? Why. Message-ID: <3cca9997-97f7-7c49-9531-4069618539b1@denninger.net>
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From the man page for mdconfig(8)
-o [no]option
Set or reset options.
[no]async
For vnode backed devices: avoid IO_SYNC for increased
performance but at the risk of deadlocking the entire
kernel.
Why and how severe is this risk?
The scenario where I care is the use of md to build and populate a
"disk" image often used with Crochet and nanobsd so as to create an
image file that can then be directly dd'd to a media device (e.g. SD
card) and booted.
When the backing store for this is a ZFS file on spinning rust the
performance is beyond horrid; about 50tps is all you get, presumably due
to clashes between the backing store and md itself which winds up
forcing a crazy amount of seeks. I suspect its seek-related because if
the zfs pool is on SSD its plenty fast -- but a backing on spinning
rust, including all mirror vdev-comprised pools (which are quite fast
for general file storage) is silly-slow.
Setting the zfs mountpoint to async has no impact on performance. I
assume that setting mdconfig's option very well might, but kernel
deadlocks are bad news, so I'm trying to understand the risk profile
since these builds are done on a production machine.
Thanks!
--
Karl Denninger
karl@denninger.net
/The Market Ticker/
/[S/MIME encrypted email preferred]/
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<p>From the man page for mdconfig(8)</p>
<p> -o [no]option<br>
Set or reset options.<br>
<br>
[no]async<br>
For vnode backed devices: avoid IO_SYNC for
increased<br>
performance but at the risk of deadlocking
the entire<br>
kernel.<br>
</p>
<p>Why and how severe is this risk?</p>
<p>The scenario where I care is the use of md to build and populate
a "disk" image often used with Crochet and nanobsd so as to create
an image file that can then be directly dd'd to a media device
(e.g. SD card) and booted.</p>
<p>When the backing store for this is a ZFS file on spinning rust
the performance is beyond horrid; about 50tps is all you get,
presumably due to clashes between the backing store and md itself
which winds up forcing a crazy amount of seeks. I suspect its
seek-related because if the zfs pool is on SSD its plenty fast --
but a backing on spinning rust, including all mirror
vdev-comprised pools (which are quite fast for general file
storage) is silly-slow.</p>
<p>Setting the zfs mountpoint to async has no impact on
performance. I assume that setting mdconfig's option very well
might, but kernel deadlocks are bad news, so I'm trying to
understand the risk profile since these builds are done on a
production machine.</p>
<p>Thanks!<br>
</p>
<div class="moz-signature">-- <br>
Karl Denninger<br>
<a href="mailto:karl@denninger.net" class="moz-txt-link-freetext">karl@denninger.net</a><br>
<i>The Market Ticker</i><br>
<font size="-2"><i>[S/MIME encrypted email preferred]</i></font></div>
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