[Drbd-dev] [PATCH 03/23] sd: implement REQ_OP_WRITE_ZEROES

[Martin K. Petersen]

Bart Van Assche <Bart.VanAssche at sandisk.com> writes:

Hi Bart,

> A quote from SBC: "An OPTIMAL UNMAP GRANULARITY field set to a
> non-zero value indicates the optimal granularity in logical blocks for
> unmap requests (e.g., an UNMAP command or a WRITE SAME (16) command
> with the UNMAP bit set to one).  An unmap request with a number of
> logical blocks that is not a multiple of this value may result in
> unmap operations on fewer LBAs than requested."

Indeed. Fewer LBAs than requested may be *unmapped*. That does not imply
that they are not *written*.

> This means that just like the start and end of a discard must be
> aligned on a discard_granularity boundary, WRITE SAME commands with
> the UNMAP bit set must also respect that granularity. I think this
> means that either __blkdev_issue_zeroout() has to be modified such
> that it rejects unaligned REQ_OP_WRITE_ZEROES operations or that
> blk_bio_write_same_split() has to be modified such that it generates
> REQ_OP_WRITEs for the unaligned start and tail.

No, that's not correct. SBC states:

"a) if the Data-Out Buffer of the WRITE SAME command is the same as the
   logical block data returned by a read operation from that LBA while
   in the unmapped state, then:

   1) the device server performs the actions described in table 6
      [unmap]; and

   2) if an unmap operation is not performed in step 1), then the device
      server shall perform the specified write operation to that LBA;"

I.e. With WRITE SAME it is the responsibility of the device server to
write any LBAs described by the command that were not successfully
unmapped.

-- 
Martin K. Petersen	Oracle Linux Engineering