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Re: [dm-devel] [PATCH 3/3] dm-thin: fix discard_granularity



On Tue, Jul 17, 2012 at 03:35:04PM -0400, Mikulas Patocka wrote:
> 
> 
> On Tue, 17 Jul 2012, Mike Snitzer wrote:
> 
> > On Mon, Jul 16 2012 at  2:35pm -0400,
> > Mikulas Patocka <mpatocka 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 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 am not sure I understand completely what you are trying to say. But
after paolo's patch, blkdev_issue_discard() will take into account
max_discard_sectors to limit max discard request size and use
discard_granularity and discard_alignment to determine aligned request start.

First request in the range will go as it is and can be unaligned but if
discard range is big, then rest of the request start will be aligned.

Because there might be an unligned requests at the start of range drivers
will still have to handle unaligned requests, i think.

Thanks
Vivek


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