Re: [PATCH 4/7][TAKE5] support new modes in fallocate

David Chinner
2007-06-26T23:18:33Z

On Tue, Jun 26, 2007 at 11:34:13AM -0400, Andreas Dilger wrote:
> On Jun 26, 2007 16:02 +0530, Amit K. Arora wrote:
> > On Mon, Jun 25, 2007 at 03:46:26PM -0600, Andreas Dilger wrote:
> > > Can you clarify - what is the current behaviour when ENOSPC (or some other
> > > error) is hit? Does it keep the current fallocate() or does it free it?
> >
> > Currently it is left on the file system implementation. In ext4, we do
> > not undo preallocation if some error (say, ENOSPC) is hit. Hence it may
> > end up with partial (pre)allocation. This is inline with dd and
> > posix_fallocate, which also do not free the partially allocated space.
>
> Since I believe the XFS allocation ioctls do it the opposite way (free
> preallocated space on error) this should be encoded into the flags.
> Having it "filesystem dependent" just means that nobody will be happy.

No, XFs does not free preallocated space on error. it is up to the
application to clean up.

> What I mean is that any data read from the file should have the "appearance"
> of being zeroed (whether zeroes are actually written to disk or not). What
> I _think_ David is proposing is to allow fallocate() to return without
> marking the blocks even "uninitialized" and subsequent reads would return
> the old data from the disk.

Correct, but for swap files that's not an issue - no user should be able
too read them, and FA_MKSWAP would really need root privileges to execute.

Cheers,

Dave.
--
Dave Chinner
Principal Engineer
SGI Australian Software Group