Re: [PATCH 1/6] fs: add hole punching to fallocate

Ted Ts'o
2010-11-09T03:29:23Z

On Tue, Nov 09, 2010 at 12:12:22PM +1100, Dave Chinner wrote:
> Hole punching was not included originally in fallocate() for a
> variety of reasons. IIRC, they were along the lines of:
>
> 1 de-allocating of blocks in an allocation syscall is wrong.
> People wanted a new syscall for this functionality.
> 2 no glibc interface needs it
> 3 at the time, only XFS supported punching holes, so there
> is not need to support it in a generic interface
> 4 the use cases presented were not considered compelling
> enough to justify the additional complexity (!)
>
> In the end, I gave up arguing for it to be included because just
> getting the FALLOC_FL_KEEP_SIZE functionality was a hard enough
> battle.
>
> Anyway, #3 isn't the case any more, #4 was just an excuse not to
> support anything ext4 couldn't do and lots of apps are calling
> fallocate directly (because glibc can't use FALLOC_FL_KEEP_SIZE) so
> #2 isn't an issue, either.

I don't recall anyone arguing #4 because of ext4, but I get very tired
of the linux-fsdevel bike-shed painting parties, so I often will
concede whatever is necessary just to get the !@#! interface in,
assuming we could add more flags later....

glibc does support fallocate(), BTW; it's just posix_fallocate() that
doesn't use FALLOC_FL_KEEP_SIZE.

> I guess that leaves #1 to be debated;
> I don't think there is any problem with doing what you propose.

I don't have a problem either.

As a completely separate proposal, what do people think about an
FALLOCATE_FL_ZEROIZE after which time the blocks are allocated, but
reading from them returns zero. This could be done either by (a)
sending a discard in the case of devices where discard_zeros_data is
true and discard_granularty is less than the fs block size, or (b) by
setting the uninitialized flag in the extent tree.

- Ted