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

To: David Chinner <dgc@xxxxxxx>
Subject: Re: [PATCH 2/9]: Reduce Log I/O latency
From: Matt Mackall <mpm@xxxxxxxxxxx>
Date: Wed, 21 Nov 2007 20:57:27 -0600
Cc: Andi Kleen <andi@xxxxxxxxxxxxxx>, xfs-oss <xfs@xxxxxxxxxxx>, lkml <linux-kernel@xxxxxxxxxxxxxxx>
In-reply-to: <20071122011214.GR114266761@xxxxxxx>
References: <20071122003339.GH114266761__34694.2978365861$1195691722$gmane$org@xxxxxxx> <p73oddnhzoq.fsf@xxxxxxxxxxxxxx> <20071122011214.GR114266761@xxxxxxx>
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On Thu, Nov 22, 2007 at 12:12:14PM +1100, David Chinner wrote:
> 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.

I've seen PVRs that would be upset by this. They put media on one
filesystem and database/apps/swap/etc. on another, but have everything
on a single spindle. Stalling a media filesystem read for a write
anywhere else = fail.

-- 
Mathematics is the supreme nostalgia of our time.


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