why block number changed after change of file?

[Zhang Shukun <bitzsk gmail com>]

hi,

i touch a new file and input the content "a" to it, next i use filefrag
to see the block number of the file , and then open the file to change
the content of the file to "aa", next use filefrag to check the block
number again, but why block number changed after change of file? is the
block number the hard-disk block number or memory block number? if it
is memory block number ,how to get the unchangable hard-disk block
number?

Thanks!

the commands displays as follow:

zsk zsk-laptop:~$ sudo filefrag -v a
Checking a
Filesystem type is: ef53
Filesystem cylinder groups is approximately 104
Blocksize of file a is 4096
File size of a is 3 (1 blocks)
First block: 2501741
Last block: 2501741
a: 1 extent found
zsk zsk-laptop:~$ vim a
zsk zsk-laptop:~$ sudo filefrag -v a
Checking a
Filesystem type is: ef53
Filesystem cylinder groups is approximately 104
Blocksize of file a is 4096
File size of a is 4 (1 blocks)
First block: 3013843
Last block: 3013843
a: 1 extent found
-- 
Regards,
Sucan