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Date: Fri, 19 Apr 2002 09:08:14 +0100
To: Stephen Lord <lord@sgi.com>
Cc: Andrew Morton <akpm@zip.com.au>, Alan Cox <alan@lxorguk.ukuu.org.uk>,
        Mark Peloquin <peloquin@us.ibm.com>, linux-kernel@vger.kernel.org
Subject: Re: Bio pool & scsi scatter gather pool usage
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In-Reply-To: <OFCF00F1A4.2665039D-ON85256B9F.006B755C@pok.ibm.com> <E16yLS4-0005vN-00@the-village.bc.nu> <3CBF5B67.E488A8E5@zip.com.au> <3CBFC755.50106@sgi.com>
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From: Joe Thornber <joe@fib011235813.fsnet.co.uk>
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On Fri, Apr 19, 2002 at 02:29:25AM -0500, Stephen Lord wrote:
> But this gets you lowest common denominator sizes for the whole
> volume, which is basically the buffer head approach, chop all I/O up
> into a chunk size we know will always work. Any sort of nasty  boundary
> condition at one spot in a volume means the whole thing is crippled
> down to that level. It then becomes a black magic art to configure a
> volume which is not restricted to a small request size.

This is exactly the problem; I don't think it's going to be unusual to
see volumes that have a variety of mappings.  For example the
'journal' area of the lv with a single fast pv, 'small file' area with
a linear mapping across normal pv's, and finally a 'large file' area
that has a few slower disks striped together.

The last thing I want in this situation is to split up all the io into
the lowest common chunk size, in this case the striped area which will
typically be  < 64k.

LVM and EVMS need to do the splitting and resubmitting of bios
themselves.

- Joe
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