On Fri, Jul 02 2010 at 4:00pm -0400, Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote: > > > On Fri, 2 Jul 2010, Mikulas Patocka wrote: > > > > As we discussed, we have a challenge where we need DM to avoid issuing > > > a barrier before the discard IFF a target doesn't support the discard > > > (which the barrier is paired with). > > > > > > My understanding is that blkdev_issue_discard() only cares if the > > > discard was supported. Barrier is used just to decorate the discard > > > (for correctness). So by returning -EOPNOTSUPP we're saying the discard > > > isn't supported; we're not making any claims about the implict barrier, > > > so best to avoid the barrier entirely. > > > > > > Otherwise we'll be issuing unnecessary barriers (and associated > > > performance loss). > > > > > > So yet another TODO item... Anyway: > > > > > > Acked-by: Mike Snitzer <snitzer@xxxxxxxxxx> > > > > Unnecessary barriers are issued anyway. With each freed extent. > > > > The code must issue a "SYNCHRONIZE CACHE" to flush cache for previous > > writes, then "UNMAP" and then another "SYNCHRONIZE CACHE" to commit that > > unmap to disk. And this in loop for all extents in > > "release_blocks_on_commit". > > > > One idea behind "discard barriers" was to submit a discard request and not > > wait for it. Then the request would need a barrier so that it doesn't get > > reordered with further writes (that may potentially write to the same area > > as the discarded area). But discard isn't used this way anyway, > > sb_issue_discard waits for completion, so the barrier isn't needed. > > > > Even if ext4 developers wanted asynchronous discard requests, they should > > fire all the discards at once and then submit one zero-sized barrier. Not > > barrier with each discard request. > > > > This is up to ext4 developers to optimize and remove the barriers and we > > can't do anything with it. Just send "SYNCHRONIZE > > CACHE"+"UNMAP"+"SYNCHRONIZE CACHE" like the barrier specification wants... > > > > Mikulas > > BTW. I understand that the current dm implementation will send two useless > consecutive "SYNCHRONIZE CACHE" commands discard is directed to the part > of the device that doesn't support it. Issue 1 ^^^ > But the problem is that when you use discard on a part of the device that > supports discard, it also sends two useless "SYNCHRONIZE CACHE" commands > --- they are useless for functionality, but mandated by the barrier > specification. Issue 2 ^^^ Those are 2 different issues. Please don't join them as if they are one in the same. DM should treat a discard as a first class request (which may or may not have a barrier). If a region doesn't support the discard DM has no business processing anything related to the discard (barriers included). It is as simple as that. > The fix is supposedly this: > > --- > include/linux/blkdev.h | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > Index: linux-2.6.35-rc3-fast/include/linux/blkdev.h > =================================================================== > --- linux-2.6.35-rc3-fast.orig/include/linux/blkdev.h 2010-07-02 21:59:21.000000000 +0200 > +++ linux-2.6.35-rc3-fast/include/linux/blkdev.h 2010-07-02 21:59:37.000000000 +0200 > @@ -1021,7 +1021,7 @@ static inline int sb_issue_discard(struc > block <<= (sb->s_blocksize_bits - 9); > nr_blocks <<= (sb->s_blocksize_bits - 9); > return blkdev_issue_discard(sb->s_bdev, block, nr_blocks, GFP_KERNEL, > - BLKDEV_IFL_WAIT | BLKDEV_IFL_BARRIER); > + BLKDEV_IFL_WAIT); > } > > extern int blk_verify_command(unsigned char *cmd, fmode_t has_write_perm); Hmm, older kernels use DISCARD_FL_BARRIER which merely mapped to BLKDEV_IFL_BARRIER. Seems you've stumbled onto a bug in the conversion that commit "blkdev: generalize flags for blkdev_issue_fn functions" (fbd9b09a177a481eda) performed? That commit seems to have incorrectly replaced DISCARD_FL_BARRIER with both: BLKDEV_IFL_WAIT | BLKDEV_IFL_BARRIER Dmitry and/or Jens was this intended? Mike -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
