how to get the sector numbers of a file ?

[bitzsk]

hi,

you know command filefrag can get the block numbers of a file, but how to get the sector numbers of a file ?

thanks !

[Rubberman]

I think you have to compute that yourself. If you know the starting sector of a block, and the number of sectors it encompasses, then you can determine the sectors it covers. Also, I think that in most Linux file systems, each block is a contiguous number of disc sectors. The standard Linux file systems, such as ext2/ext3 are not extent-based file systems. Sorry that I don't know enough to be more specific than this.

[bitzsk]

Thank you!

maybe the sector number can get from this formula:

multiply the block number with the block_size divide by sector_size.

[Lakshmipathi]

hi,

you know command filefrag can get the block numbers of a file, but how to get the sector numbers of a file ?

thanks !

A sector is 512 bytes and If your block size is 4096 bytes (4KB) and,Then If you are accessing say block number 1 . Then you can divide each block into 8 sectors.
(So sector 0 to 7 will represent block number 1)

Sector 8-15 will represent block 2 and so on.

multiply the block number with the block_size divide by sector_size

Yes,you will get the end sector of each block ,but not entire block .

[bitzsk]

thank you , Lakshmipathi

you gave me a very exact answer!

[Rubberman]

Thank you!

maybe the sector number can get from this formula:

multiply the block number with the block_size divide by sector_size.

IE:

sector_num = block_num * sectors_per_block
sectors_per_block = block_size / sector_size

or

sector_num = block_num * (block_size / sector_size)

FWIW, most drives use a 512 byte sector size, or at least they used to.

[bitzsk]

Hi,

i want to get the exact match between block number and sector number of a file in domU of xen. so i do follows .


root@feisty:~# ls -l
total 1
-rw-r--r-- 1 root root 4 Jun 9 08:16 a

root@feisty:~# filefrag -v a
Checking a
Filesystem type is: ef53
Filesystem cylinder groups is approximately 122
Blocksize of file a is 1024
File size of a is 2 (1 blocks)
First block: 247208
Last block: 247208
a: 1 extent found

the file has one block and block size is 247208, so the secoter size is 247208 *2 +1 == 78B51 and 247208*2 +2 == 78B52 (block number count begin from 0 , sector number count begin from 1).

root@feisty:~# echo "a" >> a

command above is a write to a file

root@feisty:~# tail /var/log/messages
Jun 9 08:15:54 feisty kernel: do_blk_req cf9d1de8: cmd cf9d1e64, sec ae85e, (2/2) buffer:cf7a1800 [write]
Jun 9 08:16:14 feisty kernel: do_blk_req cf9d152c: cmd cf9d15a8, sec ae7c6, (2/2) buffer:ce81ec00 [write]
Jun 9 08:16:19 feisty kernel: do_blk_req cf9d152c: cmd cf9d15a8, sec 2, (2/2) buffer:d06ee400 [write]
Jun 9 08:16:19 feisty kernel: do_blk_req cf9d1f40: cmd cf9d1fbc, sec 6, (2/2) buffer:d06eec00 [write]
Jun 9 08:16:19 feisty kernel: do_blk_req cf9d1e94: cmd cf9d1f10, sec 880d0, (2/2) buffer:d0392000 [write]
Jun 9 08:16:19 feisty kernel: do_blk_req cf9d1de8: cmd cf9d1e64, sec 880d4, (2/2) buffer:d0392800 [write]
Jun 9 08:16:19 feisty kernel: do_blk_req cf9d11d0: cmd cf9d124c, sec 880d8, (2/2) buffer:cecdf000 [write]
Jun 9 08:16:19 feisty kernel: do_blk_req cf9d127c: cmd cf9d12f8, sec ac002, (2/2) buffer:cf1e4400 [write]
Jun 9 08:16:19 feisty kernel: do_blk_req cf9d1124: cmd cf9d11a0, sec ae25a, (2/2) buffer:cf652400 [write]
Jun 9 08:16:19 feisty kernel: do_blk_req cf9d1b38: cmd cf9d1bb4, sec ae67c, (2/2) buffer:ce6d1800 [write]

and then to show sector number i had written, i expect to see 78B51 or 78B52 sector number. but none of then display, can you tell me why?

--
Regards,

[bitzsk]

Hi,

i want to get the exact match between block number and sector number of a file in domU of xen. so i do follows .


root@feisty:~# ls -l
total 1
-rw-r--r-- 1 root root 4 Jun 9 08:16 a

root@feisty:~# filefrag -v a
Checking a
Filesystem type is: ef53
Filesystem cylinder groups is approximately 122
Blocksize of file a is 1024
File size of a is 2 (1 blocks)
First block: 247208
Last block: 247208
a: 1 extent found

the file has one block and block size is 247208, so the secoter size is 247208 *2 +1 == 78B51 and 247208*2 +2 == 78B52 (block number count begin from 0 , sector number count begin from 1).

root@feisty:~# echo "a" >> a

command above is a write to a file

root@feisty:~# tail /var/log/messages
Jun 9 08:15:54 feisty kernel: do_blk_req cf9d1de8: cmd cf9d1e64, sec ae85e, (2/2) buffer:cf7a1800 [write]
Jun 9 08:16:14 feisty kernel: do_blk_req cf9d152c: cmd cf9d15a8, sec ae7c6, (2/2) buffer:ce81ec00 [write]
Jun 9 08:16:19 feisty kernel: do_blk_req cf9d152c: cmd cf9d15a8, sec 2, (2/2) buffer:d06ee400 [write]
Jun 9 08:16:19 feisty kernel: do_blk_req cf9d1f40: cmd cf9d1fbc, sec 6, (2/2) buffer:d06eec00 [write]
Jun 9 08:16:19 feisty kernel: do_blk_req cf9d1e94: cmd cf9d1f10, sec 880d0, (2/2) buffer:d0392000 [write]
Jun 9 08:16:19 feisty kernel: do_blk_req cf9d1de8: cmd cf9d1e64, sec 880d4, (2/2) buffer:d0392800 [write]
Jun 9 08:16:19 feisty kernel: do_blk_req cf9d11d0: cmd cf9d124c, sec 880d8, (2/2) buffer:cecdf000 [write]
Jun 9 08:16:19 feisty kernel: do_blk_req cf9d127c: cmd cf9d12f8, sec ac002, (2/2) buffer:cf1e4400 [write]
Jun 9 08:16:19 feisty kernel: do_blk_req cf9d1124: cmd cf9d11a0, sec ae25a, (2/2) buffer:cf652400 [write]
Jun 9 08:16:19 feisty kernel: do_blk_req cf9d1b38: cmd cf9d1bb4, sec ae67c, (2/2) buffer:ce6d1800 [write]

and then to show sector number i had written, i expect to see 78B51 or 78B52 sector number. but none of then display, can you tell me why?

--
Regards,

[Lakshmipathi]

Remember one thing,when append data to a file,It'll use a new data block.
Check your block number again with filefrag command.
Inode also changed when appending data,though it mainly based on the way how the application (vi ,gedit etc) open the file.

[bitzsk]

Thank you for quite reply

i am sorry i forget this fact!

so what is the best command to read a file and read all the sectors of the file? is that more command or another?