On Thu, Nov 21, 2024 at 08:04:15AM -0800, Darrick J. Wong wrote: > > IIRC it was to accommodate the test program, which presumably used > > discard for efficiency reasons because it did a lot of context switching > > to different point-in-time variations of the fs. If the discard didn't > > actually zero the range (depending on the underlying test dev), then at > > least on XFS, we'd see odd recovery issues and whatnot from the fs going > > forward/back in time. > > Yes, that's my recollection too -- performing a logwrite replay of an > old mark means that you can end up with blocks with the correct fs uuid > but an LSN that's higher than anything in the log. Recovery will then > skip the block replay, which is not correct. > > I suppose we could fix log recovery to treat incoming block LSNs that > are higher than the log head as if there were no block contents at all. > OTOH going backwards in time isn't usually a concern...right? It's probably the best we can do. Recover as far as everything validated and then give up. > > > Therefore the reason for using dm-thin was that it was an easy way to > > provide predictable behavior to the test program, where discards punch > > out blocks that subsequently return zeroes. > > Yep. The test needs to reset the block device to a zeroed state. > Discards get us there quickly, but only if discard_zeroes_data==1. > Hence bolting dm-thinp (where this is guaranteed) onto the logwrites > tests. discard_zeroes_data was unfortunately always broken as no standard gives you any such guarantee. The best you get is a guarantee that it returns zeroes if it actually deallocated the block, but if it deallocates a given block or not is a black box. > > > I don't recall all the specifics, but I thought part of the reason for > > using discard over explicit zeroing was the latter made the test > > impractically slow. I could be misremembering, but if you want to change > > it I'd suggest to at least verify runtimes on some of the preexisting > > logwrites tests as well. > > Not sure -- I think BLKZEROOUT will cause allocations and real disk > writes if we're not careful. If the device reports a queue/write_zeroes_max_bytes it supports a hardware offload. That could still write zeroes to the media if the device is stupid enough, but hopefully not many are.
