why block number changed after change of file?

[Zhang Shukun]

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 at 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 at zsk-laptop:~$ vim a
zsk at 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
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