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. For ease of debugging, patch 1 adds a sysfs entry for discard_alignment. Patch 2 adjusts the computation of the granularity-adjusted max_discard_sectors so that it prepares for the new code in patch 3, which 3 actually adjusts the split. v1->v2: added patch 1, fixed line length Paolo Bonzini (3): block: add sysfs entry for discard_alignment block: reorganize rounding of max_discard_sectors block: split discard into aligned requests block/blk-lib.c | 41 ++++++++++++++++++++++++++++------------- block/blk-sysfs.c | 11 +++++++++++ 2 files changed, 39 insertions(+), 13 deletions(-) -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
