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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: Fri, 23 Nov 2007 09:29:09 +1100
Cc: David Chinner <dgc@xxxxxxx>, Andi Kleen <andi@xxxxxxxxxxxxxx>, xfs-oss <xfs@xxxxxxxxxxx>, lkml <linux-kernel@xxxxxxxxxxxxxxx>
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On Thu, Nov 22, 2007 at 12:10:29PM -0600, Matt Mackall wrote:
> On Thu, Nov 22, 2007 at 09:31:59PM +1100, David Chinner wrote:
> [...]
> > > In other words, I/O priority is per-spindle and not per-filesystem and
> > > thus this change has consequences that leak outside the filesystem in
> > > question. That's bad.
> > 
> > This has nothing to do with this patch - it's a problem with sharing
> > a single resource in a RT system between two non-deterministic
> > constructs. e.g. I can put two ext3 filesystems on the one spindle,
> > run two completely independent RT workloads on the different
> > filesystems and have one workload DOS the other due to differences
> > in priority at the spindle.
> 
> Sure. And it's up to the RT system designer not to do something stupid
> like that. The problem is that your patch potentially promotes a
> non-RT I/O activity to an RT one without regard to the rest of the
> system.

So this:

http://marc.info/?l=linux-kernel&m=119247074517414&w=2

shouldn't be allowed, either? (rt kjournald for ext3)

> Perfectly understood. And that's fine. A system designer is allowed to
> shoot himself in the foot.

Ok. I'll point anyone that complains at you, Matt ;)

> I don't think there's any fundamental reason the I/O subsystem or
> filesystems can't be taught to handle priority inversion, which is
> much more acceptable and general fix.

See my reply to Andi.

> If I've got XFS on filesystems A and B on the same spindle (or volume
> group?) and my real RT I/O takes place only on B, then I want log
> flushing to happen in RT on B. But -never on A-. If I can do this with
> a tunable, I'm perfectly happy.

No, not another mount option. I'm just going to drop this one for
now...

Cheers,

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


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