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Date: Wed, 21 Nov 2007 20:57:27 -0600 From: Matt Mackall <mpm@...enic.com> To: David Chinner <dgc@....com> Cc: Andi Kleen <andi@...stfloor.org>, xfs-oss <xfs@....sgi.com>, lkml <linux-kernel@...r.kernel.org> Subject: Re: [PATCH 2/9]: Reduce Log I/O latency 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@....com> 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. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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