linux-kernel - Re: [PATCH 07/14] target/sbc: Add P_TYPE + PROT_EN bits to READ_CAPACITY

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Date:	Fri, 10 Jan 2014 12:15:08 -0800
From:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
To:	Andy Grover <agrover@...hat.com>
Cc:	Sagi Grimberg <sagig@...lanox.com>,
	"Nicholas A. Bellinger" <nab@...erainc.com>,
	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>,
	Christoph Hellwig <hch@....de>, Hannes Reinecke <hare@...e.de>,
	Or Gerlitz <ogerlitz@...lanox.com>
Subject: Re: [PATCH 07/14] target/sbc: Add P_TYPE + PROT_EN bits to
 READ_CAPACITY_16

On Fri, 2014-01-10 at 11:50 -0800, Andy Grover wrote:
> On 01/09/2014 10:21 PM, Nicholas A. Bellinger wrote:
> >> What about FORMAT_UNIT emulation?
> >
> > Would certainly be useful to have..
> >
> >> The backstore protection configuration is done at the target side via
> >> configfs/targetcli, if you publish DIF support in
> >> INQUERY_EVPD/READ_CAPACITY you need to accept protection information format?
> >
> > Mmmm, these two bits bits are following what scsi_debug is currently
> > exposing minus FORMAT_UNIT support..?
> >
> > MKP..?
> 
> Yes, don't you need FORMAT UNIT because protection information is going 
> to mean the pi-enabled lun will need to report less blocks?

FORMAT_UNIT is simply a mechanism that allows the client to setup the
protection information remotely, to complement the per device configfs
attribute that does the same thing from the target side.

> The ramdisk backstore changes in this series allocate extra space for
> PI info, but my understanding was that especially for emulation with
> block and fileio backstores, everything needs to go in the same amount
> of space.
> 

No, that's only for the interleaved case.

> Furthermore, if we want PI info stored along with the blocks, then block 
> and fileio backstore formats are no longer going to be 1:1 -- requiring 
> offset calculations, non-aligned read-modify-write, and all that 
> unpleasantness to be handled?
> 

I'm currently not intending to support interleaved mode into the
backend, given that backends not doing emulation expect these to be in
seperate SGLs to start.

--nab

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