[dm-devel] [PATCH 3/3] dm-thin: fix discard_granularity
Mikulas Patocka
mpatocka at redhat.com
Tue Jul 17 19:35:04 UTC 2012
On Tue, 17 Jul 2012, Mike Snitzer wrote:
> On Mon, Jul 16 2012 at 2:35pm -0400,
> Mikulas Patocka <mpatocka at redhat.com> wrote:
>
> > dm-thin: fix discard_granularity
> >
> > The kernel expects that limits->discard_granularity is a power of two.
> > Set this limit only if we use a power of two block size.
> >
> > Signed-off-by: Mikulas Patocka <mpatocka at redhat.com>
> >
> > ---
> > drivers/md/dm-thin.c | 3 ++-
> > 1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > Index: linux-3.5-rc6-fast/drivers/md/dm-thin.c
> > ===================================================================
> > --- linux-3.5-rc6-fast.orig/drivers/md/dm-thin.c 2012-07-16 20:07:49.000000000 +0200
> > +++ linux-3.5-rc6-fast/drivers/md/dm-thin.c 2012-07-16 20:08:01.000000000 +0200
> > @@ -2502,7 +2502,8 @@ static void set_discard_limits(struct po
> > * bios cover a block partially. A discard that spans a block boundary
> > * is not sent to this target.
> > */
> > - limits->discard_granularity = pool->sectors_per_block << SECTOR_SHIFT;
> > + if (pool->sectors_per_block_shift >= 0)
> > + limits->discard_granularity = pool->sectors_per_block << SECTOR_SHIFT;
> > limits->discard_zeroes_data = pool->pf.zero_new_blocks;
> > }
>
> Given the block layer's assumption that discard_granularity is always a
> power of 2: thinp should disable discard if the thinp blocksize is a non
> power of 2. So this patch isn't correct (discard support should be
> disabled in pool_ctr based on the specified blocksize).
discard_granularity is just a hint (and IMHO quite useless hint).
The documentation says that it indicates a size of internal allocation
unit that may be larger than the block size. The code doesn't use it this
way - it is used in FITRIM ioctl where it specifies the minimum request
size to be sent. It is also used in blkdev_issue_discard where it is used
to round down the number of sectors to discard on discard_granularity
boundary - this is wrong, it aligns request size on discard_granularity
boundary, but it doesn't align request start on this boundary.
I don't see a reason why we should disable discard because of an a rarely
used hint.
Mikulas
> Future work should be:
> 1) explore/eliminate the block layer's power of 2 constraints
> 2) divorce thinp's discard limits (for discard passdown) from the pool's
> blocksize
>
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