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

To: Andi Kleen <andi@xxxxxxxxxxxxxx>
Subject: Re: [PATCH 2/9]: Reduce Log I/O latency
From: David Chinner <dgc@xxxxxxx>
Date: Thu, 22 Nov 2007 12:12:14 +1100
Cc: David Chinner <dgc@xxxxxxx>, xfs-oss <xfs@xxxxxxxxxxx>, lkml <linux-kernel@xxxxxxxxxxxxxxx>
In-reply-to: <p73oddnhzoq.fsf@xxxxxxxxxxxxxx>
References: <20071122003339.GH114266761__34694.2978365861$1195691722$gmane$org@xxxxxxx> <p73oddnhzoq.fsf@xxxxxxxxxxxxxx>
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On Thu, Nov 22, 2007 at 01:49:25AM +0100, Andi Kleen wrote:
> David Chinner <dgc@xxxxxxx> 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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