Re: [PATCH 03/13] scsi: unify allocation of scsi command and sensebuffer

From: Matthew Wilcox
Date: Tue May 26 2009 - 11:14:01 EST

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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