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

To: Matt Mackall <mpm@xxxxxxxxxxx>
Subject: Re: [PATCH 2/9]: Reduce Log I/O latency
From: David Chinner <dgc@xxxxxxx>
Date: Thu, 22 Nov 2007 14:41:06 +1100
Cc: David Chinner <dgc@xxxxxxx>, Andi Kleen <andi@xxxxxxxxxxxxxx>, xfs-oss <xfs@xxxxxxxxxxx>, lkml <linux-kernel@xxxxxxxxxxxxxxx>
In-reply-to: <20071122025726.GG17536@xxxxxxxxx>
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On Wed, Nov 21, 2007 at 08:57:27PM -0600, Matt Mackall wrote:
> On Thu, Nov 22, 2007 at 12:12:14PM +1100, David Chinner wrote:
> > 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.

Sounds like the PVR is badly designed to me. If a write can cause a
read to miss a playback deadline, then you haven't built enough
buffering into your playback application.


Dave Chinner
Principal Engineer
SGI Australian Software Group

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