How many 4096 byte blocks on disk?

[glitch003]

Hi, I want to find out how many 4096 byte blocks there are on my disk. I used df -B 4096 and it gave me a number but I'm not sure it's correct as I can use dd to read past what should be the final block.

So I do df -B 4096 and it reports this result:

Filesystem           4K-blocks      Used Available Use% Mounted on
/dev/sda1             15618840  13190294   1635137  89% /

But when I use dd to go past that block, it doesn't report an error or anything. The command I'm using is

dd if=/dev/sda1 bs=4096 count=1 skip=15618841

How can I know that I'm really reading the very last block on the drive?

Thanks!!

[saikee]

I think if sda1 has some overhead, like the filing indexing system etc, occupying the front end and the actual file storage is shorter than the length of sda1.

Here is my own example. My sda1 occupies from block 1 to 15400th block (1 block=1 cylinder=8225280 byte).

saikee@saikee-desktop:~$ sudo fdisk -l /dev/sda
[sudo] password for saikee: 

Disk /dev/sda: 128.0 GB, 128035676160 bytes
255 heads, 63 sectors/track, 15566 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0xde00600d

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *           1       15400   123700468+  83  Linux
/dev/sda2           15401       15566     1333364+   5  Extended
/dev/sda5           15401       15566     1333363+  82  Linux swap / Solaris

However if I do a df I get

saikee@saikee-desktop:~$ df -B 8225280
Filesystem         8.3MB-blocks      Used Available Use% Mounted on
/dev/sda1                15159      3449     10940  24% /
none                       381         1       381   1% /dev
none                       382         1       381   1% /dev/shm
none                       382         1       382   1% /var/run
none                       382         0       382   0% /var/lock
none                       382         0       382   0% /lib/init/rw
/dev/sdb58               43453     13157     28088  32% /media/7f5e7b46-3579-415f-83f5-8b2e79a6c9c7
/dev/sdb58               43453     13157     28088  32% /mnt/sdb58
/dev/sdb3                59844     41042     15763  73% /mnt/sdb3

The file storage length of my sda1 is only 15159 blocks. This suggests the overhead section of the partition is 241 block long or 15% of the partition space.

When I using dd to display the contents of sda1 it complaints if the number of block exceeds the physical limit.

saikee@saikee-desktop:~$ sudo dd if=/dev/sda1 bs=8225280 count=1 skip=15400
dd: `/dev/sda1': cannot skip: Invalid argument
0+0 records in
0+0 records out
0 bytes (0 B) copied, 6.3942e-05 s, 0.0 kB/s

However I get one block displayed by dd if I skip 15399 blocks indicating the physical limit of the partition.

Thus the maximum block size you see in df does not corresponds to the end of the partition.

[glitch003]

This is exactly what I wanted to know. Thank you very much!!