On Tue, Jan 12, 2021 at 11:51:07AM -0700, Andreas Dilger wrote: > On Jan 12, 2021, at 11:43 AM, Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote: > > > > On Tue, Jan 12, 2021 at 11:39:58AM -0700, Andreas Dilger wrote: > >>> 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. > >>> > >>>> 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 > > > > >
