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

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Date:	Sun, 12 Jan 2014 13:49:36 +0200
From:	Sagi Grimberg <sagig@....mellanox.co.il>
To:	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Andy Grover <agrover@...hat.com>
CC:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>,
	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>,
	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 1/10/2014 10:46 PM, Martin K. Petersen wrote:
>>>>>> "Andy" == Andy Grover <agrover@...hat.com> writes:
> Andy> Yes, don't you need FORMAT UNIT because protection information is
> Andy> going to mean the pi-enabled lun will need to report less blocks?
>
> Modern disk drives won't shrink when you reformat them with PI. This is
> a result of an IDEMA agreement about LBA counts.
>
> And if you create a 10GB PI LUN on an array you'll get 10GB for data.
>
> Andy> The ramdisk backstore changes in this series allocate extra space
> Andy> for PI info, but my understanding was that especially for
> Andy> emulation with block and fileio backstores, everything needs to go
> Andy> in the same amount of space.
>
> For both file and block I'd recommend we store the PI in a separate
> block device or file unless the backing device is PI-capable.
>
> Andy> Furthermore, if we want PI info stored along with the blocks, then
> Andy> block and fileio backstore formats are no longer going to be 1:1
> Andy> -- requiring offset calculations, non-aligned read-modify-write,
> Andy> and all that unpleasantness to be handled?
>
> I only think interleaved makes sense if you're passing the PI through
> instead of emulating.
>

I agree, I implemented interleaved mode just as a proof of concept that 
our HW can perform offload in that manner.
I assume we can stick with non-interleaved, although it can be added as 
a user option.

Sagi.
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