Date: Thu, 24 Jan 2008 23:47:39 -0700
From: Andreas Dilger <adilger@sun.com>
Subject: Re: [RFC] ext3: per-process soft-syncing data=ordered mode
In-reply-to: <200801242336.00340.a1426z@gawab.com>
To: Al Boldi <a1426z@gawab.com>
Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Mail-followup-to: Al Boldi <a1426z@gawab.com>, linux-fsdevel@vger.kernel.org,
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Message-id: <20080125064739.GS18433@webber.adilger.int>
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On Jan 24, 2008  23:36 +0300, Al Boldi wrote:
> data=ordered mode has proven reliable over the years, and it does this by 
> ordering filedata flushes before metadata flushes.  But this sometimes 
> causes contention in the order of a 10x slowdown for certain apps, either 
> due to the misuse of fsync or due to inherent behaviour like db's, as well 
> as inherent starvation issues exposed by the data=ordered mode.
> 
> data=writeback mode alleviates data=order mode slowdowns, but only works 
> per-mount and is too dangerous to run as a default mode.
> 
> This RFC proposes to introduce a tunable which allows to disable fsync and 
> changes ordered into writeback writeout on a per-process basis like this:
> 
>       echo 1 > /proc/`pidof process`/softsync

If fsync performance is an issue for you, run the filesystem in data=journal
mode, put the journal on a separate disk and make it big enough that you
don't block on it to flush the data to the filesystem (but not so big that
it is consuming all of your RAM).

That keeps your data guarantees without hurting performance.

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.

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