linux-kernel - Re: [PATCH 2/9]: Reduce Log I/O latency

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Message-ID: <20071122011214.GR114266761@sgi.com>
Date:	Thu, 22 Nov 2007 12:12:14 +1100
From:	David Chinner <dgc@....com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	David Chinner <dgc@....com>, xfs-oss <xfs@....sgi.com>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/9]: Reduce Log I/O latency

On Thu, Nov 22, 2007 at 01:49:25AM +0100, Andi Kleen wrote:
> David Chinner <dgc@....com> writes:
> 
> > To ensure that log I/O is issued as the highest priority I/O, set
> > the I/O priority of the log I/O to the highest possible. This will
> > ensure that log I/O is not held up behind bulk data or other
> > metadata I/O as delaying log I/O can pause the entire transaction
> > subsystem. Introduce a new buffer flag to allow us to tag the log
> > buffers so we can discrimiate when issuing the I/O.
> 
> Won't that possible disturb other RT priority users that do not need 
> log IO (e.g. working on preallocated files)? Seems a little
> dangerous.

In all the cases that I know of where ppl are using what could
be considered real-time I/O (e.g. media environments where they
do real-time ingest and playout from the same filesystem) the
real-time ingest processes create the files and do pre-allocation
before doing their I/O. This I/O can get held up behind another
process that is not real time that has issued log I/O. 

Given there is no I/O priority inheritence and having log I/O stall
will stall the entire filesystem, we cannot allow log I/O to
stall in real-time environments. Hence it must have the highest
possible priority to prevent this.

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group
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