On 04/01/2021 21.10, Andres Freund wrote:
Hi,
On 2021-01-04 10:19:58 -0800, Darrick J. Wong wrote:
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 ...
Doing so as a variant of FALLOC_FL_ZERO_RANGE seems to make the most
sense, as that'd work reasonably efficiently to initialize newly
allocated space as well as for zeroing out previously used file space.
As blkdev_issue_zeroout() already has a fallback path it seems this
should be doable without too much concern for which devices have write
zeroes, and which do not?
Question: do you want the kernel to write zeroes even for devices that
don't support accelerated zeroing?
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.
But note it will need to be plumbed down to md and dm to be generally
useful.
