On Tue, Jan 12, 2021 at 11:51:07AM -0700, Andreas Dilger wrote:

On Jan 12, 2021, at 11:43 AM, Christoph Hellwig [off-list ref] wrote:

quoted

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

quoted

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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).

I'm not against adding XFS_IOC_ALLOCSP64 to ext4, if applications are actually
using that.

<shudder> Some of them are, but--

ALLOCSP64 can only allocate pre-zeroed blocks as part of extending EOF,
whereas a new FZERO flag means that we can pre-zero an arbitrary range
of bytes in a file.  I don't know if Avi or Andres' usecases demand that
kind of flexibilty but I know I'd rather go for the more powerful
interface.

--D

It also makes sense to me that there also be an fallocate() mode for allocating
zeroed blocks (which was the original request), since fallocate() is already
doing very similar things and is the central interface for managing block
allocation instead of having a filesystem-specific ioctl() to do this.

Cheers, Andreas