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

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Date:	Thu, 15 Nov 2012 12:31:15 -0800
From:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
To:	"Elliott, Robert (Server Storage)" <Elliott@...com>
Cc:	Christoph Hellwig <hch@....de>,
	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

Hi Robert,

On Thu, 2012-11-15 at 20:01 +0000, Elliott, Robert (Server Storage)
wrote:
> 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.
> 

Thanks for the heads up here..  I'm making the slight change to the -v2
patch series that just went out to always check the max_write_same_len
value for both WRITE_SAME w/ UNMAP=[1,0] cases.

Thank you,

--nab


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