linux-kernel - Re: [PATCH] btrfs: always write superblocks synchronously

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Date:   Wed, 3 May 2017 10:55:44 -0400
From:   Chris Mason <clm@...com>
To:     Jan Kara <jack@...e.cz>, Davidlohr Bueso <dave@...olabs.net>
CC:     <dsterba@...e.com>, <linux-btrfs@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <stable@...r.kernel.org>,
        Jan Kara <jack@...e.cz>, Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH] btrfs: always write superblocks synchronously



On 05/03/2017 04:36 AM, Jan Kara wrote:
> On Tue 02-05-17 09:28:13, Davidlohr Bueso wrote:
>> Commit b685d3d65ac7 "block: treat REQ_FUA and REQ_PREFLUSH as
>> synchronous" removed REQ_SYNC flag from WRITE_FUA implementation.
>> Since REQ_FUA and REQ_FLUSH flags are stripped from submitted IO
>> when the disk doesn't have volatile write cache and thus effectively
>> make the write async. This was seen to cause performance hits up
>> to 90% regression in disk IO related benchmarks such as reaim and
>> dbench[1].
>>
>> Fix the problem by making sure the first superblock write is also
>> treated as synchronous since they can block progress of the
>> journalling (commit, log syncs) machinery and thus the whole filesystem.
>>

>>
>> Fixes: b685d3d65ac (block: treat REQ_FUA and REQ_PREFLUSH as synchronous)
>> Cc: stable <stable@...r.kernel.org>
>> Cc: Jan Kara <jack@...e.cz>
>> Signed-off-by: Davidlohr Bueso <dbueso@...e.de>
>
> I wasn't patient enough and already sent the fix as part of my series
> fixing other filesystems [1]. It also fixes one more place in btrfs that
> needs REQ_SYNC to return to the original behavior.
>


Thanks guys.

-chris