Re: scsi: convert discard to REQ_TYPE_FS instead of REQ_TYPE_BLOCK_PC

[Mike Snitzer <snitzer@redhat.com>]

On Fri, Jul 09 2010 at 12:27pm -0400,
Martin K. Petersen <martin.petersen@oracle.com> wrote:

> >>>>> "Mike" == Mike Snitzer <snitzer@redhat.com> writes:
> 
> Mike> Turns out that this LUN has a 4K granularity requirement (will
> Mike> fail the WRITE SAME if the granularity requirements are not met).
> 
> That's lame.  The devices I've been working with just ignore the
> portions of the block range that are not aligned / do not constitute
> entire blocks.

What is really lame is this LUN also doesn't have the correct
discard_granularity!

# cat /sys/block/sda/queue/discard_granularity 
512
 
> I've had the following patch kicking around in my tree for a while.  It
> does not yet handle offset discard alignment, though.

OK, won't help us for this LUN but thanks.

I'm left wondering whether the higher level that is discarding a
particular large region (say blkdev_issue_discard's loop) should also be
trained about discard alignment too.  That way the discard requests are
submitted properly aligned and they aren't reduced (leaving holes, like
your patch could do) -- except for the last discard in a sequence, which
could be reduced if its granularity is wrong.

No idea if the above will make sense to others ;)

Mike