Re: [PATCH 07/14] target/sbc: Add P_TYPE + PROT_EN bits to READ_CAPACITY_16

[Sagi Grimberg <sagig@dev.mellanox.co.il>]

On 1/10/2014 10:46 PM, Martin K. Petersen wrote:
>>>>>> "Andy" == Andy Grover <agrover@redhat.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.