Re: fallocate(FALLOC_FL_ZERO_RANGE_BUT_REALLY) to avoid unwritten extents?

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:

quoted

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.

Since I assume that if the fallocate fails you'll fall back to writing
zeroes from userspace anyway...

And there's non-linux platforms as well, at least that's the rumor I hear.

Second question: Would it help to have a FALLOC_FL_DRY_RUN flag that
could be used to probe if a file supports fallocate without actually
changing anything?  I'm (separately) pursuing a fix for the loop device
not being able to figure out if a file actually supports a particular
fallocate mode.

Hm. I can see some potential uses of such a flag, but I haven't really
wished for it so far.

Greetings,

Andres Freund