Hi, On 2021-01-04 10:19:58 -0800, Darrick J. Wong wrote: > On Tue, Dec 29, 2020 at 10:28:19PM -0800, Andres Freund wrote: > > Would it make sense to add a variant of FALLOC_FL_ZERO_RANGE that > > doesn't convert extents into unwritten extents, but instead uses > > blkdev_issue_zeroout() if supported? Mostly interested in xfs/ext4 > > myself, but ... > > > > Doing so as a variant of FALLOC_FL_ZERO_RANGE seems to make the most > > sense, as that'd work reasonably efficiently to initialize newly > > allocated space as well as for zeroing out previously used file space. > > > > > > As blkdev_issue_zeroout() already has a fallback path it seems this > > should be doable without too much concern for which devices have write > > zeroes, and which do not? > > Question: do you want the kernel to write zeroes even for devices that > don't support accelerated zeroing? I don't have a strong opinion on it. A complex userland application can do a bit better job managing queue depth etc, but otherwise I suspect doing the IO from kernel will win by a small bit. And the queue-depth issue presumably would be relevant for write-zeroes as well, making me lean towards just using the fallback. > Since I assume that if the fallocate fails you'll fall back to writing > zeroes from userspace anyway... And there's non-linux platforms as well, at least that's the rumor I hear. > Second question: Would it help to have a FALLOC_FL_DRY_RUN flag that > could be used to probe if a file supports fallocate without actually > changing anything? I'm (separately) pursuing a fix for the loop device > not being able to figure out if a file actually supports a particular > fallocate mode. Hm. I can see some potential uses of such a flag, but I haven't really wished for it so far. Greetings, Andres Freund
