linux-kernel - Re: [RFC] ext3: per-process soft-syncing data=ordered mode

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Message-ID: <20080125153634.GG28856@atrey.karlin.mff.cuni.cz>
Date:	Fri, 25 Jan 2008 16:36:34 +0100
From:	Jan Kara <jack@...e.cz>
To:	Al Boldi <a1426z@...ab.com>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC] ext3: per-process soft-syncing data=ordered mode

> Greetings!
> 
> 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
  I guess disabling fsync() was already commented on enough. Regarding
switching to writeback mode on per-process basis - not easily possible
because sometimes data is not written out by the process which stored
them (think of mmaped file). And in case of DB, they use direct-io
anyway most of the time so they don't care about journaling mode anyway.
  But as Diego wrote, there is definitely some room for improvement in
current data=ordered mode so the difference shouldn't be as big in the
end.

								Honza
-- 
Jan Kara <jack@...e.cz>
SuSE CR Labs
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