linux-ext4 - Re: [PATCH] jbd2: Fix dbench4 performance regression for 'nobarrier' mounts

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Date:   Sat, 29 Apr 2017 21:14:18 -0400
From:   Theodore Ts'o <tytso@....edu>
To:     Jan Kara <jack@...e.cz>
Cc:     linux-ext4@...r.kernel.org, Christoph Hellwig <hch@...radead.org>,
        stable@...r.kernel.org
Subject: Re: [PATCH] jbd2: Fix dbench4 performance regression for 'nobarrier'
 mounts

On Fri, Apr 28, 2017 at 11:59:34AM +0200, Jan Kara wrote:
> Commit b685d3d65ac7 "block: treat REQ_FUA and REQ_PREFLUSH as
> synchronous" removed REQ_SYNC flag from WRITE_FUA implementation. Since
> JBD2 strips REQ_FUA and REQ_FLUSH flags from submitted IO when the
> filesystem is mounted with nobarrier mount option, journal superblock
> writes ended up being async writes after this patch and that caused
> heavy performance regression for dbench4 benchmark with high number of
> processes. In my test setup with HP RAID array with non-volatile write
> cache and 32 GB ram, dbench4 runs with 8 processes regressed by ~25%.
> 
> Fix the problem by making sure journal superblock writes are always
> treated as synchronous since they generally block progress of the
> journalling machinery and thus the whole filesystem.
> 
> Fixes: b685d3d65ac791406e0dfd8779cc9b3707fea5a3
> CC: stable@...r.kernel.org
> Signed-off-by: Jan Kara <jack@...e.cz>

Thanks, applied.

						- Ted