On Jan 12, 2021, at 11:16 AM, Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote: > > On Mon, Jan 04, 2021 at 09:57:48PM +0200, Avi Kivity wrote: >>> 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. >>> >> >> The new flag will avoid requiring DMA to transfer the entire file size, and >> perhaps can be implemented in the device by just adjusting metadata. So >> there is potential for the new flag to be much more efficient. > > We already support a WRITE_ZEROES operation, which many (but not all) > NVMe devices and some SCSI devices support. The blkdev_issue_zeroout > helper can use those, or falls back to writing actual zeroes. > > 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. Cheers, Andreas
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