linux-kernel - RE: [PATCH 3/3] target/iblock: Add WRITE

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Date:	Thu, 15 Nov 2012 20:01:27 +0000
From:	"Elliott, Robert (Server Storage)" <Elliott@...com>
To:	Christoph Hellwig <hch@....de>,
	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
CC:	Christoph Hellwig <hch@...radead.org>,
	target-devel <target-devel@...r.kernel.org>,
	linux-scsi <linux-scsi@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"Martin K. Petersen" <martin.petersen@...cle.com>
Subject: RE: [PATCH 3/3] target/iblock: Add WRITE_SAME w/ UNMAP=0 emulation
	support

WRITE SAME always has a payload, regardless of the UNMAP bit value.

For WRITE SAME with UNMAP=0, it's extremely important; that's how what to write is specified.

For WRITE SAME with UNMAP=1, the device server is required to check that the payload matches the data that is returned for unmapped LBAs.  lf LBPRZ=1 (read zeros for unmapped LBAs), that means checking that the payload has all zeros.  In sbc3r33, this rule is tucked away in model section 4.7.3.4.3, not the command section 5.41.

I would like to change that rule (it's a nuisance and a performance burden), but that's the current rule going into SBC-3 letter ballot.  

Changing WRITE SAME with UNMAP=1 to ignore the payload would provide essentially the same functionality as changing the UNMAP command to be mandatory, not just a hint; both approaches have been discussed.

> -----Original Message-----
> From: linux-scsi-owner@...r.kernel.org [mailto:linux-scsi-
> owner@...r.kernel.org] On Behalf Of Christoph Hellwig
> Sent: Thursday, 15 November, 2012 1:33 PM
> To: Nicholas A. Bellinger
> Cc: Christoph Hellwig; target-devel; linux-scsi; linux-kernel; Christoph Hellwig;
> Martin K. Petersen
> Subject: Re: [PATCH 3/3] target/iblock: Add WRITE_SAME w/ UNMAP=0
> emulation support
> 
> On Thu, Nov 15, 2012 at 11:29:46AM -0800, Nicholas A. Bellinger wrote:
> > Well at least for the latter that is because UNMAP=0 does not have a
> > payload.  ;)
> 
> UNMAP=0 does have a payload, we just ignore it.  In fact I was told
> that targets should check for a completely zeroed sector sized payload
> for it if being pedantic.  I can't really find the justification for
> that in the standard - the closest thing to it is the stance about
> ignoring the unmap bit if the device is fully provisioned.
> 
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