linux-kernel - Re: [PATCH 8/8] vm: Add an tuning knob for vm.max_writeback

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Message-ID: <4AA633FD.3080006@gmail.com>
Date:	Tue, 08 Sep 2009 13:37:49 +0300
From:	Artem Bityutskiy <dedekind1@...il.com>
To:	Jens Axboe <jens.axboe@...cle.com>
CC:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	chris.mason@...cle.com, david@...morbit.com, hch@...radead.org,
	akpm@...ux-foundation.org, jack@...e.cz,
	"Theodore Ts'o" <tytso@....edu>
Subject: Re: [PATCH 8/8] vm: Add an tuning knob for vm.max_writeback_mb

Hi,

On 09/08/2009 12:23 PM, Jens Axboe wrote:
> From: Theodore Ts'o<tytso@....edu>
>
> Originally, MAX_WRITEBACK_PAGES was hard-coded to 1024 because of a
> concern of not holding I_SYNC for too long.  (At least, that was the
> comment previously.)  This doesn't make sense now because the only
> time we wait for I_SYNC is if we are calling sync or fsync, and in
> that case we need to write out all of the data anyway.  Previously
> there may have been other code paths that waited on I_SYNC, but not
> any more.
>
> According to Christoph, the current writeback size is way too small,
> and XFS had a hack that bumped out nr_to_write to four times the value
> sent by the VM to be able to saturate medium-sized RAID arrays.  This
> value was also problematic for ext4 as well, as it caused large files
> to be come interleaved on disk by in 8 megabyte chunks (we bumped up
> the nr_to_write by a factor of two).
>
> So, in this patch, we make the MAX_WRITEBACK_PAGES a tunable,
> max_writeback_mb, and set it to a default value of 128 megabytes.
>
> http://bugzilla.kernel.org/show_bug.cgi?id=13930
>
> Signed-off-by: "Theodore Ts'o"<tytso@....edu>
> Signed-off-by: Jens Axboe<jens.axboe@...cle.com>

Would be nice to update doc files like

Documentation/sysctl/vm.txt
Documentation/filesystems/proc.txt

as well.

-- 
Best Regards,
Artem Bityutskiy (Артём Битюцкий)
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