On Tue, Jun 26, 2007 at 11:34:13AM -0400, Andreas Dilger wrote:

On Jun 26, 2007  16:02 +0530, Amit K. Arora wrote:

quoted

On Mon, Jun 25, 2007 at 03:46:26PM -0600, Andreas Dilger wrote:

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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.

Ok, got your point. Maybe we can have a flag for this, as you suggested.
But, default behavior IMHO should be _not_ to undo partial allocation
(thus the file system will have the option of supporting this flag or
not and it will be inline with posix_fallocate; XFS will obviously
like to support this flag, inline with its existing behavior).

quoted

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For FA_ZERO_SPACE - I'd think this would (IMHO) be the default - we
don't want to expose uninitialized disk blocks to userspace.  I'm not
sure if this makes sense at all.

I don't think we need to make it default - atleast for filesystems which
have a mechanism to distinguish preallocated blocks from "regular" ones.

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.

I can't think of a good reason for this (i.e. returning stale data from
preallocated blocks). It is infact a security issue to me.
Anyhow, this may though be beneficial for file systems which have
noticable overhead in marking the blocks "uninitialized/preallocated".
Can you or David please throw some light on how this option might really
be helpful ? Thanks!

--
Regards,
Amit Arora