[Linux] Ext3 partition detected wrongfully as ext4

14 November 2008

Oh No!

Recently, A partition on my backup harddrive was corrupted. It was i was clueless how it had happened. further, now the partition was refusing to mount and it was being detected as ext4.

I tried mounting is manually:

gaurish  ~  $   sudo mount /dev/sdb6 /media/fl/
mount: unknown filesystem type 'ext4'

Dmesg said

“EXT3-fs: sdb6: couldn’t mount because of unsupported optional features (ca9bc1e0).”

I was not sure what to do next

So i cried for help on various online Forums[1][2][3] but all in waste.

After that i got pissed off and decided to take matter into my own hands and starting reading on how Partitions in linux are made etc. Then i came to know about superblock.

Bingo!, i had a bad superblock.

Here is a Quick Procedure to recover from a bad super block(read nightmare)

Open Terminal and type the following commands:

1) see backup superblock location

`~ $ dumpe2fs /dev/sdb6 grep -i superblock

Output:

dumpe2fs 1.41.3 (12-Oct-2008)

Primary superblock at 0, Group descriptors at 1-2

Backup superblock at 32768, Group descriptors at 32769-32770

Backup superblock at 98304, Group descriptors at 98305-98306

Backup superblock at 163840, Group descriptors at 163841-163842

Backup superblock at 229376, Group descriptors at 229377-229378

Backup superblock at 294912, Group descriptors at 294913-294914

Backup superblock at 819200, Group descriptors at 819201-819202

Backup superblock at 884736, Group descriptors at 884737-884738

Backup superblock at 1605632, Group descriptors at 1605633-1605634

Backup superblock at 2654208, Group descriptors at 2654209-2654210

Backup superblock at 4096000, Group descriptors at 4096001-4096002

2) Run Fsck with Alternative Superblock

$ sudo fsck -b 32768 /dev/sdb6

fsck 1.41.3 (12-Oct-2008)

e2fsck 1.41.3 (12-Oct-2008)

/dev/sdb6 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 #12 (30380, counted=30381).

Fix? yes

Free blocks count wrong for group #127 (15027, counted=5927).

Fix? yes

Free blocks count wrong (10539272, counted=10530173).

Fix? yes

Free inodes count wrong for group #116 (32732, counted=32729).

Fix? yes

Free inodes count wrong (24812957, counted=24812954).

Fix? yes

/dev/sdb6: *** FILE SYSTEM WAS MODIFIED ***

/dev/sdb6: 934/24813888 files (6.2% non-contiguous), 14282211/24812384 blocks

3) Done! Partition is successfully recovered and data is safe. now we can mount it normally.

$ sudo mount /dev/sdb6 /media/test

Now, After this incident i have to rethink my backup strategy, which till now have been reply solely on HDDs as backup medium.

Comment & Suggestions are welcome :)

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