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SubjectRe: [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
headers.

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

Tested-by: Mike Snitzer <snitzer@redhat.com>


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