On Jun 27, 2007  09:14 +1000, David Chinner wrote:

Someone on the XFs list had an interesting request - preallocated
swap files. You can't use unwritten extents for this because
of sys_swapon()s use of bmap() (XFS returns holes for reading
unwritten extents), so we need a method of preallocating that does
not zero or mark the extent unread. i.e. FA_MKSWAP.

Is there a reason why unwritten extents return 0 to bmap()?  This
would seem to be the only impediment from using fallocated files
for swap files.  Maybe if FIEMAP was used by mkswap to get an
"UNWRITTEN" flag back instead of "HOLE" it wouldn't be a problem.

That way we can allocate large swap files that don't need zeroing
in a single, fast operation, and hence potentially bring new
swap space online without needed very much memory at all (i.e.
should succeed in most near-OOM conditions).

Cheers, Andreas
--
Andreas Dilger
Principal Software Engineer
Cluster File Systems, Inc.