linux-kernel - Re: [patch 0/1] Apply segment size and segment boundary to integrity data

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Message-ID: <4C3F3A0C.10909@kernel.dk>
Date:	Thu, 15 Jul 2010 10:40:44 -0600
From:	Jens Axboe <axboe@...nel.dk>
To:	"Martin K. Petersen" <martin.petersen@...cle.com>
CC:	Christof Schmitt <christof.schmitt@...ibm.com>,
	linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 0/1] Apply segment size and segment boundary to integrity
 data

On 07/15/2010 10:35 AM, Martin K. Petersen wrote:
>>>>>> "Jens" == Jens Axboe <axboe@...nel.dk> writes:
> 
> Jens> That sounds like a very batch design decision. Either the limits
> Jens> are explicitly given and different, or if not we have to assume
> Jens> that they are the same as the data limits at least.
> 
> Imagine a controller that has a 4KB segment, 1 entry limit.  If we
> capped the DI sgl the same way as the data we'd only be able to issue
> 512-byte requests unless the DI entries happened to be contiguous in
> memory.
> 
> For several types of I/O the DI sgl is much longer than the data sgl.
> Especially if the submitter is using buffer_heads to map 512-byte
> blocks.
> 
> And consequently we require vendors to be able to handle the
> pathological case in which any data scatterlist honoring the
> segmentation constraints given by the driver can be matched with an
> integrity scatterlist in which there is a separate entry for each
> logical block.  No vendor has had any problems with this.  Therefore
> there are no block layer data integrity queue limits.
> 
> If a device appears that does in fact have constraints I have no
> problems intruducing a set of suitable knobs.

OK, lets wait and hear what problem that Christof ran into here.
Is it ensuring that a segment doesn't corss eg the 4GB boundary?

-- 
Jens Axboe

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