Re: fallocate(FALLOC_FL_PUNCH_HOLE)

[Richard Laager <rlaager@wiktel.com>]

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On Wed, 2012-03-14 at 14:27 +1100, Dave Chinner wrote:
> On Sat, Mar 10, 2012 at 02:07:05PM -0600, Richard Laager wrote:
> > If the answer to #2 is not C, it would appear there's no atomic way to
> > indicate that I'm done with certain data* but I want the filesystem to
> > continue to guarantee space for me. Is this correct?
> 
> Not through fallocate() right now. XFS has an ioctl that will turn
> written ranges and holes back into preallocated space:
> XFS_IOC_ZERO_RANGE.

Do filesystems generally track the data necessary to tell the difference
between fallocate() + write() and just a regular write()? If so, it
might be nice for applications to be able to say "I'm done with this
data" and effectively "undo" the write(). In other words, the space
would return to being unallocated or preallocated, whichever it was
originally.

I suspect they don't track preallocation of data ranges once they're
filled with data. So, for example, QEMU will have to be told whether the
administrator wants thin (i.e. use PUNCH_HOLE) or thick (i.e. use
ZERO_RANGE) provisioning.

-- 
Richard

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