LinuxHostingSupport

A SuperBlock in Linux saves information about the File System like, the File System type, size, status etc. A File system cannot be mounted if a Superblock is corrupted.  Corruption of superblock can occur due to various reasons like, abnormal shutdown due to power failure, virus infection, file system corruption etc.

When a Superblock is corrupted, you receive a ” can’t read superblock” error message while accessing the File System. For example, if you try to access a Linux ext3 partition say, /dev/sda3, you will receive the following message:

/dev/sda3: Input/output error
mount: /dev/sda3: can’t read superblock

Linux ext3 file system automatically maintains a backup of superblock at various locations. In cases such as these, you have to restore a superblock from an alternate  backup location to retrieve the data.

Note: You should unmount the partition before performing this task.

First, find / list the superblock locations of the file system /dev/sda3 (we are using sda3 as an example, your partition may be different)

# dumpe2fs /dev/sda3 | grep superblock
 dumpe2fs 1.39 (29-May-2006)
 Primary superblock at 1, Group descriptors at 2-2
 Backup superblock at 8193, Group descriptors at 8194-8194
 Backup superblock at 24577, Group descriptors at 24578-24578
 Backup superblock at 40961, Group descriptors at 40962-40962
 Backup superblock at 57345, Group descriptors at 57346-57346
 Backup superblock at 73729, Group descriptors at 73730-73730

Now, check and repair (fsck) the file system with an alternate superblock #24577. BTW, try superblock from another location if one doesn’t work.

# fsck -b 24577 /dev/sda3
 fsck 1.39 (29-May-2006)
 e2fsck 1.39 (29-May-2006)
 /dev/sda3 was not cleanly unmounted, check forced.
 Pass 1: Checking inodes, blocks, and sizes
 Pass 2: Checking directory structure
 Pass 3: Checking directory connectivity
 Pass 4: Checking reference counts
 Pass 5: Checking group summary information
 Free blocks count wrong for group #0 (3553, counted=513).
 Fix<y>? yes

Free blocks count wrong for group #1 (7681, counted=5059).
 Fix<y>? yes

Free blocks count wrong for group #19 (7939, counted=7697).
 Fix<y>? yes

/boot: ***** FILE SYSTEM WAS MODIFIED *****
 /boot: 35/50200 files (8.6% non-contiguous), 17906/200780 blocks

Now, mount the partition once the file system check is over:

# mount /dev/sda3 /mnt

Once the partition is mounted, you can retrieve the files from /mnt:

# mkdir backup
# cd /mnt
# cp filename /backup/

BTW, it is always good to keep a backup of your data instead of finding yourself in such situations.