On Thu, Jul 05 2012 at 12:01pm -0400,
Paolo Bonzini [off-list ref] wrote:

When a disk has a large discard_granularity, discards are not split with
optimal alignment; the pessimization gets bigger as discard_granularity
and max_discard_sectors become closer.

Take the limit case of discard_granularity == max_discard_sectors == 64.
Then, if a request is submitted for 256 sectors 2..257 it will be split
like this: 2..65, 66..129, 130..193, 194..257.  None of these requests
is aligned, so in fact you might end up with no discarded logical blocks
at all.  With this patch, the split will be 2..63, 64..127, 128..191,
192..255, 256..257.  The patches also take the discard_alignment into
consideration.

Patch 1 adjusts the computation of the granularity-adjusted
max_discard_sectors so that it prepares for the new code in patch 2,
which actually adjusts the split.

v2->v3: drop addition of queue/discard_alignment to sysfs, use
  correct alignment for partitions

Paolo Bonzini (2):
  block: reorganize rounding of max_discard_sectors
  block: split discard into aligned requests

 block/blk-lib.c        |   41 ++++++++++++++++++++++++++++-------------
 include/linux/blkdev.h |   10 ++++++++++
 2 files changed, 38 insertions(+), 13 deletions(-)

Hey Jens,

Would be great to get these discard fixes in.  I know both Christoph and
Vivek have reviewed these changes but that isn't reflected in the patch
headers.

These patches eliminate misaligned discard from being sent to the
dm-thinp target.

Tested-by: Mike Snitzer <redacted>

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