Re: TAKE 972756 - Implement fallocate.

[David Chinner <dgc@sgi.com>]

On Tue, Nov 06, 2007 at 10:57:03PM +0530, Bhagi rathi wrote:
> File is of size 1k.  A 4k block is allocated as file-system block size is
> 4k.
>  Preallocation happened from 1k to 256k. Now, it looks to me that we have
> un-written extents from 4k to 256k. There is no guarantee that data from 1k
> to 4k is all zero'es. Fallocate is updating size.  Hence on subsequent read,
> we can get garbage from 1k to 4k and all zero'es from 4k to 256k

# rm /mnt/test/fred
# xfs_io -f -c "pwrite 0 1024" -c "fsync" -c "falloc_allocsp 0 262144" -c "bmap -vp" /mnt/test/fred
wrote 1024/1024 bytes at offset 0
1 KiB, 1 ops; 0.0000 sec (42.459 MiB/sec and 43478.2609 ops/sec)
/mnt/test/fred:
 EXT: FILE-OFFSET      BLOCK-RANGE      AG AG-OFFSET        TOTAL FLAGS
   0: [0..7]:          14520..14527      0 (14520..14527)       8 00000
   1: [8..511]:        345688..346191    0 (345688..346191)   504 10000
# dd if=/mnt/test/fred bs=4k count=1 |od -Ax
1+0 records in
1+0 records out
4096 bytes (4.1 kB) copied, 0.004566 seconds, 897 kB/s
000000 146715 146715 146715 146715 146715 146715 146715 146715
*
000400 000000 000000 000000 000000 000000 000000 000000 000000
*
001000

Only 1k of modified data, then 3k of zeros, then a bunch of unwritten extents
out to EOF.

Cheers,

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