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Re: [dm-devel] [PATCH v3 0/2] block: improvements for discard alignment

On Thu, Jul 05 2012 at 12:01pm -0400,
Paolo Bonzini <pbonzini redhat com> 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

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

Tested-by: Mike Snitzer <snitzer redhat com>

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