Hi,

On 2021-01-07 09:52:01 +1100, Dave Chinner wrote:

On Tue, Dec 29, 2020 at 10:28:19PM -0800, Andres Freund wrote:

quoted

Which brings me to $subject:

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 ...

We have explicit requests from users (think initialising large VM
images) that FALLOC_FL_ZERO_RANGE must never fall back to writing
zeroes manually.

That behaviour makes a lot of sense for quite a few use cases - I wasn't
trying to make it sound like it should not be available. Nor that
FALLOC_FL_ZERO_RANGE should behave differently.

IOWs, while you might want FALLOC_FL_ZERO_RANGE to explicitly write
zeros, we have users who explicitly don't want it to do this.

Right - which is why I was asking for a variant of FALLOC_FL_ZERO_RANGE
(jokingly named FALLOC_FL_ZERO_RANGE_BUT_REALLY in the subject), rather
than changing the behaviour.

Perhaps we should add want FALLOC_FL_CONVERT_RANGE, which tells the
filesystem to convert an unwritten range of zeros to a written range
by manually writing zeros. i.e. you do FALLOC_FL_ZERO_RANGE to zero
the range and fill holes using metadata manipulation, followed by
FALLOC_FL_WRITE_RANGE to then convert the "metadata zeros" to real
written zeros.

Yep, something like that would do the trick. Perhaps
FALLOC_FL_MATERIALIZE_RANGE?

Greetings,

Andres Freund