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

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Message-ID: <4A1C14F3.5090209@panasas.com>
Date:	Tue, 26 May 2009 19:12:35 +0300
From:	Boaz Harrosh <bharrosh@...asas.com>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
CC:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	jens.axboe@...cle.com, rdreier@...co.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 05/26/2009 05:47 PM, James Bottomley wrote:
> 
> 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).

This one is not possible because it is scsi-ml in majority of cases that
does the DMA request through scsi_eh_prep_cmnd() and a regular read.
The drivers don't even know anything about it.

>      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).
> 

This is the way to go for sure. And only on ARCHs with none-coherent-cache
all the good ARCHs can just use embedded sense just fine.

> I'd need a little more clarity on the actual size of the problem before
> making any choices.
> 
> The other thing to bear in mind is that two allocations of M and N might
> be more costly than a single allocation of N+M; however, an allocation
> of M+N+extra can end up more costly if the extra causes more page
> reclaim before we get an actual command.
> 
> James
> 
Boaz
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