linux-kernel - Re: [PATCH 03/13] scsi: unify allocation of scsi command and sense buffer

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Date:	Tue, 26 May 2009 09:13:46 -0600
From:	Matthew Wilcox <matthew@....cx>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
Cc:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	jens.axboe@...cle.com, rdreier@...co.com, bharrosh@...asas.com,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	chris.mason@...cle.com, david@...morbit.com, hch@...radead.org,
	akpm@...ux-foundation.org, jack@...e.cz,
	yanmin_zhang@...ux.intel.com, linux-scsi@...r.kernel.org
Subject: Re: [PATCH 03/13] scsi: unify allocation of scsi command and sense
	buffer

On Tue, May 26, 2009 at 09:47:02AM -0500, James Bottomley wrote:
> > Yeah, we can inline the sense buffer but as we discussed in the past
> > several times, there are some good reasons that we should not do so, I
> > think.
> 
> There are several other approaches:
> 
>      1. Keep the sense buffer packed in the command but disallow DMA to
>         it, which fixes all the alignment problems.  Then we supply a
>         set of rotating DMA buffers to drivers which need to do the DMA
>         (which isn't the majority).
>      2. Sense is a comparative rarity, so us a more compact pooling
>         scheme and discard sense for reuse as soon as we know it's not
>         used (as in at softirq time when there's no sense collected).
> 
> I'd need a little more clarity on the actual size of the problem before
> making any choices.

I'm not sure if this is what you meant by option 2 or not, but one
proposal was to keep a number of sense buffers around per-host, and only
allocate extras when we run close to empty.

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."
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