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

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Date:	Mon, 8 Nov 2010 22:30:38 -0500
From:	Ted Ts'o <tytso@....edu>
To:	Dave Chinner <david@...morbit.com>
Cc:	Josef Bacik <josef@...hat.com>, linux-kernel@...r.kernel.org,
	linux-btrfs@...r.kernel.org, linux-ext4@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, xfs@....sgi.com,
	joel.becker@...cle.com, cmm@...ibm.com, cluster-devel@...hat.com
Subject: Re: [PATCH 1/6] fs: add hole punching to fallocate

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
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