linux-kernel - Re: Scatter-gather list constraints

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Date:	Thu, 26 Jun 2008 15:00:34 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	stern@...land.harvard.edu, linux-kernel@...r.kernel.org,
	antonio.lin@...ormicro.com, david.vrabel@....com
Subject: Re: Scatter-gather list constraints

On Thu, Jun 26 2008, Andi Kleen wrote:
> 
> > On the fs side, just setting a hw block size of 1k should fix the
> > problem, since that'd be your minimum transfer size AND alignment there
> > even for O_DIRECT IO.
> 
> XFS used to force 512 byte IOs for its log IO. Not sure
> that was ever fixed. If it was fixed it likely required a disk format
> change (I think s390 ran into a problem like this)

Issuing IO less than the hardware block size is illegal, so if they do
that then they can't be supported on hardware with > 512b block sizes.
Someone has to do the RMW for such an operation and we don't do it in
drivers.

-- 
Jens Axboe

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