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Date: Tue, 28 Nov 2023 11:29:51 +0100
To: cygwin AT cygwin DOT com
Subject: Re: Cygwin api to punch a hole into a file?
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From: Corinna Vinschen via Cygwin <cygwin AT cygwin DOT com>
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On Nov 24 12:01, Corinna Vinschen via Cygwin wrote:
> On Nov 23 23:36, Cedric Blancher via Cygwin wrote:
> > Linux has fallocate(fd, FALLOC_FL_PUNCH_HOLE|FALLOC_FL_KEEP_SIZE, ...)
> > to punch a hole into a file, i.e. deallocate the blocks given and make
> > the file a "sparse file".
> 
> We don't support the Linux-specific fallocate(2) call, only ftruncate(2)
> and posix_fallocate(3).  Patches, as usual, thoughtfully considered.

The next test release cygwin-3.5.0-0.485.g65831f88d6c4 introduces
the Linux-specific fallocate(2) call.  Naturally we can't support
all flags, but the following flag combinations are allowed:

- 0				same as posix_fallocate(3)
- FALLOC_FL_KEEP_SIZE
- FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE
- FALLOC_FL_ZERO_RANGE
- FALLOC_FL_ZERO_RANGE | FALLOC_FL_KEEP_SIZE

A few comments:

- With 0 and FALLOC_FL_KEEP_SIZE, sparse blocks in the given range
  are re-allocated as per the POSIX requirements.  For that, it uses
  the same code as FALLOC_FL_ZERO_RANGE, but only for the holes within
  the range.

- With FALLOC_FL_KEEP_SIZE, over-allocation is done by setting
  the allocation size of a file while keeping EOF the same.
  However, in contrast to your typical Linux filesystem, over-
  allocation on Windows filesystems is only temporary.  The
  over-allocated blocks are returned to free blocks as soon as
  the last HANDLE to the file is closed.

- With FALLOC_FL_KEEP_SIZE, no over-allocation is performed on sparse
  files.  The reason is that over-allocation on sparse files has no
  effect on Windows.

- FALLOC_FL_ZERO_RANGE is implemented as writing zeros to the
  given range.  For parts of the range which are already allocated
  data blocks, as much zeros are written as necessary.  For holes
  in sparse files, only a single zero byte is written to the
  hole per 64K chunk, which is the allocation granularity of
  sparse files.

- FALLOC_FL_ZERO_RANGE is NOT atomic.

Please give it a try.


Corinna

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