Subject: Re: Limiting disk I/O?'
To: Michael van Elst <firstname.lastname@example.org>
From: Bill Stouder-Studenmund <email@example.com>
Date: 12/04/2007 13:20:20
Content-Type: text/plain; charset=us-ascii
On Tue, Dec 04, 2007 at 08:40:28PM +0000, Michael van Elst wrote:
> firstname.lastname@example.org (Matthias Scheler) writes:
> >> Now guess what happens when you temporarily bypass softdep's ordering.
> >Bad things(TM)
> Why would you want to stop disk writes? The kernel can easily _read_
> data (when it has free buffers) even when it pushes out dirty
> buffers in a tight loop. The problem is that you don't have
> free buffers because all are occupied by data that needs to
> be written. Stopping the writes won't free any buffer.
The problem is that our experience does not agree with your assertion=20
about the kernel being able to read while writing. I think you're correct=
that the kernel can easily issue reads while it is writing in a tight=20
loop. The problem is getting the reads actually serviced by the drive.
The reason to stop writing is to let other access happen. Also, part of=20
the idea is to avoid the avalanche by starting to write sooner and pacing=
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