On Thu, 24 Sep 2020, John Dorminy wrote: > I don't understand how this works... > > Can chunk_size_bytes be 0? If not, how is discard_granularity being set to 0? Yes - chunk_size_bytes is 0 here. > I think also limits is local to the ti in question here, initialized > by blk_set_stacking_limits() via dm-table.c, and therefore has only > default values and not anything to do with the underlying queue. So In dm_calculate_queue_limits, there's blk_set_stacking_limits(limits); - that will initialize the default limits (discard_granularity == 0) Then, there's: ti->type->iterate_devices(ti, dm_set_device_limits, &ti_limits); - that will unify limits for all RAID legs (in this case, it sets discard_granularity == 1024) Then, there's if (ti->type->io_hints) ti->type->io_hints(ti, &ti_limits); And that will incorrectly overwrite discard_granularity with zero. > setting discard_granularity=max(discard_granularity, chunk_size_bytes) > doesn't seem like it should be working, unless I'm not understanding > what it's there for... > > And shouldn't melding in the target's desired io_hints into the > existing queue limits be happening in blk_stack_limits() instead? > (Also, it does lcm_not_zero() for stacking granularity, instead of > max()...) Well, you can add blk_stack_limits to raid_io_hints and use it to stack the limits, but that would just complicate that function. Mikulas -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
