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

David Chinner
2007-06-26T23:15:07Z

On Mon, Jun 25, 2007 at 03:46:26PM -0600, Andreas Dilger wrote:
> On Jun 25, 2007 20:33 +0530, Amit K. Arora wrote:
> > I have not implemented FA_FL_FREE_ENOSPC and FA_ZERO_SPACE flags yet, as
> > *suggested* by Andreas in http://lkml.org/lkml/2007/6/14/323 post.
> > If it is decided that these flags are also needed, I will update this
> > patch. Thanks!
>
> 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?
>
> 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.

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.

I thinkthis would be:

#define FA_FL_NO_ZERO_SPACE 0x08 /* default is to zero space */

#define FA_MKSWAP (FA_ALLOCATE | FA_FL_NO_ZERO_SPACE)

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,

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