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

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Date:	Thu, 22 Nov 2007 14:28:43 +1100
From:	Stewart Smith <stewart@...ql.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, 2007-11-22 at 12:12 +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.

FWIW from a "real time" database POV this seems to make sense to me...
in fact, we probably rely on filesystem metadata way too much
(historically it's just "worked".... although we do seem to get issues
on ext3).

I have a (casually stupid) simulation program... although I've observed
little to no problems on all my XFS tests using it.

-- 
Stewart Smith, Senior Software Engineer (MySQL Cluster)
MySQL AB, www.mysql.com
Office: +14082136540 Ext: 6616
VoIP: 6616@....us.mysql.com
Mobile: +61 4 3 8844 332

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