linux-kernel - Re: Abysmal I/O scheduling with dm-crypt

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Date:	Wed, 7 Sep 2011 22:51:43 -0400
From:	Dan Merillat <dan.merillat@...il.com>
To:	Jan Kara <jack@...e.cz>
Cc:	martin f krafft <madduck@...duck.net>,
	linux kernel mailing list <linux-kernel@...r.kernel.org>
Subject: Re: Abysmal I/O scheduling with dm-crypt

> On Tue 30-08-11 09:14:07, martin f krafft wrote:
>> People not running dm-crypt seem unable to reproduce this problem,
>> making me think that it must be due to dm-crypt, and I wouldn't find
>> it hard to imagine, because dm-crypt basically shields the
>> I/O-scheduler of the kernel, doesn't it? Worse, it probably doesn't
>> make any effort of scheduling I/O itself. Note: I know very little
>> about the internals, so please correct me if I am wrong.

No, it's not specific to dm-crypt, these kinds of annoying IO hangs happen
on bare drives, MD arrays or DM-* setups.

There's a number of causes - including firefox history updates and the
abuse of create/sync/rename due to metadata hitting disk before data.

There's not going to be any movement on that front, I'm afraid, see
"Don't fear the fsync" (reproduced at
http://www.linuxfoundation.org/news-media/blogs/browse/2009/03/don%E2%80%99t-fear-fsync
 , the original site couldn't be reached while I'm writing this).

You can either throw more hardware at it, or use libeatmydata to
disable fsync across the board.  Maybe, eventually, a filesystem will
be written that can manage an atomic replace without fsync().  Don't
hold your breath.
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