> -----Original Message----- > From: Martin K. Petersen [mailto:[email protected]] > Sent: Wednesday, January 07, 2015 4:19 PM > To: Sam Bradshaw (sbradshaw) > Cc: Martin K. Petersen; [email protected]; [email protected] > Subject: Re: [PATCH] block: pass correct seed to integrity metadata > generation function > > >>>>> "Sam" == Sam Bradshaw (sbradshaw) <[email protected]> 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.
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