inside the mind of a linux admin

Linux: Recover Corrupted Partition From A Bad Superblock

So you have a drive that’s gone bad, and you cannot mount it. How can you recover a bad superblock from a corrupted ext3 partition to get back your data? Linux ext2/3 filesystem stores superblocks at different locations so it is possible to recover data from a corrupted partition. Your error will likely look similar to this:

#mount /dev/sda2
/dev/sda2: Input/output error
mount: /dev/sda2: can’t read superblock

or

# mount /dev/sdb1 /mnt
mount: special device /dev/sdb1 does not exist

Note: Your device may be different, I provided two separate examples from an sda partition and sdb partition, but check your server and become familiar with its’ drives before messing with your file system at all.

Mount partition using alternate superblock

Find out superblock location for /dev/sda2:

# dumpe2fs /dev/sda2 | grep superblock

WARNING! Make sure file system is UNMOUNTED.

If your system will give you a terminal type the following command, else boot Linux system from rescue disk (boot from 1st CD/DVD. At boot: prompt type command linux rescue)

Sample output:

Primary superblock at 0, Group descriptors at 1-6
Backup superblock at 32768, Group descriptors at 32769-32774
Backup superblock at 98304, Group descriptors at 98305-98310
Backup superblock at 163840, Group descriptors at 163841-163846
Backup superblock at 229376, Group descriptors at 229377-229382
Backup superblock at 294912, Group descriptors at 294913-294918
Backup superblock at 819200, Group descriptors at 819201-819206
Backup superblock at 884736, Group descriptors at 884737-884742
Backup superblock at 1605632, Group descriptors at 1605633-1605638
Backup superblock at 2654208, Group descriptors at 2654209-2654214
Backup superblock at 4096000, Group descriptors at 4096001-4096006
Backup superblock at 7962624, Group descriptors at 7962625-7962630
Backup superblock at 11239424, Group descriptors at 11239425-11239430
Backup superblock at 20480000, Group descriptors at 20480001-20480006
Backup superblock at 23887872, Group descriptors at 23887873-23887878

Now check and repair a Linux file system using alternate superblock # 32768 on /dev/sda2:

# fsck -b 32768 /dev/sda2

Sample output:

fsck 1.40.2 (12-Jul-2007)
e2fsck 1.40.2 (12-Jul-2007)
/dev/sda2 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 #241 (32254, counted=32253).
Fix? yes

Free blocks count wrong for group #362 (32254, counted=32248).
Fix? yes

Free blocks count wrong for group #368 (32254, counted=27774).
Fix? yes
……….
/dev/sda2: ***** FILE SYSTEM WAS MODIFIED *****
/dev/sda2: 59586/30539776 files (0.6% non-contiguous), 3604682/61059048 blocks

Now try to mount file system using mount command:

# mount /dev/sda2 /mnt

You can also use superblock stored at 32768 to mount partition, enter:

# mount sb={alternative-superblock} /dev/device /mnt
# mount sb=32768 /dev/sda2 /mnt

Now , try to browse and access file system:

# cd /mnt
# mkdir test
# ls -l
# cp file /path/to/safe/location

Hopefully, this article will allow you to recover the critical data you need, and that you’ve learned your lesson on THE IMPORTANCE OF BACKUPS.

😉

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