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    SubjectRe: [PATCH 4/7][TAKE5] support new modes in fallocate
    On Jun 26, 2007  16:15 +0530, Amit K. Arora wrote:
    > On Mon, Jun 25, 2007 at 03:52:39PM -0600, Andreas Dilger wrote:
    > > In XFS one of the (many) ALLOC modes is to zero existing data on allocate.
    > > For ext4 all this would mean is calling ext4_ext_mark_uninitialized() on
    > > each extent. For some workloads this would be much faster than truncate
    > > and reallocate of all the blocks in a file.
    >
    > In ext4, we already mark each extent having preallocated blocks as
    > uninitialized. This is done as part of following code (which is part of
    > patch 5/7) in ext4_ext_get_blocks() :

    What I meant is that with XFS_IOC_ALLOCSP the previously-written data
    is ZEROED OUT, unlike with fallocate() which leaves previously-written
    data alone and only allocates in holes.

    So, if you had a sparse file with some data in it:

    AAAAA BBBBBB

    fallocate() would allocate the holes:

    00000AAAAA000000000BBBBBB00000000

    XFS_IOC_ALLOCSP would overwrite everything:

    000000000000000000000000000000000

    In order to specify this for allocation, FA_FL_DEL_DATA would need to make
    sense for allocations (as well as the deallocation). This is farily easy
    to do - just mark all of the existing extents as unallocated, and their
    data disappears.

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

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