On Tue, Jan 12, 2021 at 11:39:58AM -0700, Andreas Dilger wrote:

quoted

XFS already has a XFS_IOC_ALLOCSP64 that is defined to actually
allocate written extents.  It does not currently use
blkdev_issue_zeroout, but could be changed pretty trivially to do so.

quoted

But note it will need to be plumbed down to md and dm to be generally
useful.

DM and MD already support mddev_check_write_zeroes, at least for the
usual targets.

Similarly, ext4 also has EXT4_GET_BLOCKS_CREATE_ZERO that can allocate zero
filled extents rather than unwritten extents (without clobbering existing
data like FALLOC_FL_ZERO_RANGE does), and just needs a flag from fallocate()
to trigger it.  This is plumbed down to blkdev_issue_zeroout() as well.

XFS_IOC_ALLOCSP64 actually is an ioctl that has been around since 1995
on IRIX (as an fcntl).