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


    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).


    Dave Chinner
    Principal Engineer
    SGI Australian Software Group
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