RE: [PATCH] block: pass correct seed to integrity metadata generation function

[Sam Bradshaw (sbradshaw)]

> -----Original Message-----
> From: Martin K. Petersen [mailto:martin.petersen@xxxxxxxxxx]
> Sent: Wednesday, January 07, 2015 4:19 PM
> To: Sam Bradshaw (sbradshaw)
> Cc: Martin K. Petersen; axboe@xxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH] block: pass correct seed to integrity metadata
> generation function
> 
> >>>>> "Sam" == Sam Bradshaw (sbradshaw) <sbradshaw@xxxxxxxxxx> writes:
> 
> Sam> Yes.
> 
> The seed is just a seed. We happen to set it to the (block layer)
> sector number if nothing else is provided but it's essentially just an
> incrementing counter starting at an arbitrary value chosen by the
> caller.
> 
> Since an I/O may get remapped many times as block devices are stacked
> (DM, MD, stripe splits, partition offset shifts, etc.) the seed is not
> expected to match the LBA on the storage device. That is almost never
> the case.
> 
> In SCSI we do a remapping pass before submitting a WRITE or upon
> completion of a READ. You will have to do the same for NVMe.
> 
> It was done this way to avoid remapping the ref tag several times. We
> adjust the seed as the I/O gets sliced and diced and only map the PI
> pages once to do a single traversal at the bottom of the stack.
> 
> For next gen devices we simply pass the seed to the hardware and let it
> handle the remapping. This saves us having to map the PI pages and pull
> them through the cache.

Got it, thanks!

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