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 ... One thing to note is that there are some devices which support a write zeros operation, but where it is *less* performant than actually writing zeros via DMA'ing zero pages. Yes, that's insane. Unfortunately, there are a insane devices out there.... This is not hypothetical; I know this because we tried using write zeros in mke2fs, and I got regression complaints where mke2fs/mkfs.ext4 got substantially slower for some devices. That doesn't meant that your proposal shouldn't be adopted. But it would be a good idea to have some kind of way to either allow some kind of tuning knob to disable the user of zeroout (either in the block device, file system, or in userspace), and/or some kind of way to try to automatically figure out whether using zeroout is actually a win, since most users aren't going to be up to adjusting a manual tuning knob. - Ted
