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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