On Tue, Jun 26, 2007 at 11:42:50AM -0400, Andreas Dilger wrote:
quoted text
> 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

No, it wouldn't. XFS_IOC_ALLOCSP would give you:


      AAAAA         BBBBBB00000000

because it only allocates the space between the old EOF and the new
EOF. Graphic demonstration - write 4k @ 4k, 4k @ 16k, allocsp out to 32k:

budgie:~ # xfs_io -f \
quoted text
> -c "pwrite 4096 4096" \
> -c "pwrite 16384 4096" \
> -c "bmap -vvp" \
> -c "allocsp 32768 0" \
> -c "bmap -vvp" \
> /mnt/test/alfred
wrote 4096/4096 bytes at offset 4096
4 KiB, 1 ops; 0.0000 sec (108.507 MiB/sec and 27777.7778 ops/sec)
wrote 4096/4096 bytes at offset 16384
4 KiB, 1 ops; 0.0000 sec (260.417 MiB/sec and 66666.6667 ops/sec)
/mnt/test/alfred:
 EXT: FILE-OFFSET      BLOCK-RANGE      AG AG-OFFSET          TOTAL
   0: [0..7]:          hole                                       8
   1: [8..15]:         5226864..5226871  4 (1022160..1022167)     8
   2: [16..31]:        hole                                      16
   3: [32..39]:        5226888..5226895  4 (1022184..1022191)     8
/mnt/test/alfred:
 EXT: FILE-OFFSET      BLOCK-RANGE      AG AG-OFFSET          TOTAL
   0: [0..7]:          hole                                       8
   1: [8..15]:         5226864..5226871  4 (1022160..1022167)     8
   2: [16..31]:        hole                                      16
   3: [32..63]:        5226888..5226919  4 (1022184..1022215)    32
budgie:~ #

Cheers,

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