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Linux filesystem blocks

The blocks used for two different purpose:

1. Most blocks stores user data as files (user data).
2. Some blocks in every file system store the file system’s metadata.In simple words Metadata describes the structure of the file system. Most common metadata structure are superblock, inode and directories.

Superblock

Each file system is different and they have type like ext2, ext3 etc. Further each file system has size like 5 GB, 10 GB and status such as mount status. In short each file system has a superblock, which contains information about file system such as:

* File system type
* Size
* Status
* Information about other metadata structures

If this information lost, you are in trouble (data loss) so Linux maintains multiple redundant copies of the superblock in every file system. This is very important in many emergency situation, for example you can use backup copies to restore damaged primary super block. Following command displays primary and backup superblock location on /dev/hda3

# dumpe2fs /dev/hda3 | grep -i superblock

Code:
dumpe2fs 1.35 (28-Feb-2004)
Primary superblock at 0, Group descriptors at 1-5
Backup superblock at 32768, Group descriptors at 32769-32773
Backup superblock at 98304, Group descriptors at 98305-98309
Backup superblock at 163840, Group descriptors at 163841-163845
Backup superblock at 229376, Group descriptors at 229377-229381
Backup superblock at 294912, Group descriptors at 294913-294917
Backup superblock at 819200, Group descriptors at 819201-819205
Backup superblock at 884736, Group descriptors at 884737-884741
Backup superblock at 1605632, Group descriptors at 1605633-1605637
Backup superblock at 2654208, Group descriptors at 2654209-2654213
Backup superblock at 4096000, Group descriptors at 4096001-4096005
Backup superblock at 7962624, Group descriptors at 7962625-7962629
Backup superblock at 11239424, Group descriptors at 11239425-11239429

When a file system fails then

* File system will refuse to mount
* Entire system get hangs
* Even if filesystem mount operation result into success, users may notice strange behavior when mounted such as system reboot, gibberish characters in directory listings etc

Most of time fsck (front end to ext2/ext3 utility) can fix the problem, first simply run e2fsck - to check a Linux ext2/ext3 file system, first unmount /dev/sda3 then type following command :

# e2fsck -f /dev/sda3

Where,

* -f : Force checking even if the file system seems clean.

Please note that If the superblock is not found, e2fsck will terminate with a fatal error. However Linux maintains multiple redundant copies of the superblock in every file system, so you can use -b {alternative-superblock} option to get rid of this problem.

The location of the backup superblock is dependent on the filesystem’s blocksize:

* For filesystems with 1k blocksizes, a backup superblock can be found at block 8193
* For filesystems with 2k blocksizes, at block 16384
* For 4k blocksizes, at block 32768.

Tip you can also try any one of the following command(s) to determine alternative-superblock locations:
# mke2fs -n /dev/sda3
OR
# dumpe2fs /dev/sda3|grep -i superblock

To repair file system by alternative-superblock use command as follows:
# e2fsck -f -b 8193 /dev/sda3

However it is highly recommended that you make backup before you run fsck command on system, use dd command to create a backup (provided that you have spare space under /disk2)
# dd if=/dev/sda3 of=/disk2/backup-sda3.img