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

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Date:	Tue, 01 Sep 2009 20:38:55 +0200
From:	Peter Zijlstra <peterz@...radead.org>
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,
	tytso@....edu, akpm@...ux-foundation.org, jack@...e.cz
Subject: Re: [PATCH 8/8] vm: Add an tuning knob for vm.max_writeback_pages

On Tue, 2009-09-01 at 13:19 +0200, Jens Axboe wrote:

> 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, and
> change the default to be 32768 blocks.

Do we really need a tunable for this?

I guess we need a limit to avoid it writing out everything, but can't we
have something automagic?

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