This tip will help you troubleshoot and then access damaged partitions in a Linux environment where the ext3 file system is used, as it is in about 80% of all Linux servers in use today. You'll learn how to benefit from advanced mount options that allow you to access data that you may have considered lost.
Accessing a file system with superblock
In order to access a file system, you need the superblock. This is a 1 kilobyte block that contains all metadata about the file system. It normally is the second 1 K block on an ext3 file system. In the listing below, you can see a part of the contents of the superblock as displayed with the debugfs utility:
Filesystem volume name:
Last mounted on: Filesystem UUID: 09979101-96e0-4533-a7f3-0a2db9b07a03 Filesystem magic number: 0xEF53 Filesystem revision #: 1 (dynamic) Filesystem features: has_journal ext_attr filetype needs_recovery sparse_super large_file Default mount options: (none) Filesystem state: clean Errors behavior: Continue Filesystem OS type: Linux Inode count: 5248992 Block count: 10486428 Reserved block count: 524321 Free blocks: 3888202 Free inodes: 4825213 First block: 0 Block size: 4096 Fragment size: 4096 Blocks per group: 32768 Fragments per group: 32768 Inodes per group: 16352 Inode blocks per group: 511
The problem arises when, due to some error, the superblock isn't accessible anymore. Fortunately, some backup copies of the superblock are written on the ext3 file system by default. Using these backup copies, you can still mount a file system that you may have considered lost otherwise.
The actual position on disk of the first backup of the superblock depends on the size of the file system. On modern large file systems, you will always find it at block 32768. To access it, you can use the mount option -o sb.
The problem arises when the mount expects you to specify the position of the superblock in 1024 byte blocks. The default block size for a modern ext3 volume or partition is 4096 bytes. Therefore, to tell the mount command where it can find the superblock, you have to multiply the position of the superblock by 4; which would result in the block value 131072 in most cases. If, for example, your /dev/sda5 file system had a problem, you could try mounting it with the command
mount -o sb=131072 /dev/hda5 /somewhere.
Now that you have mounted the problematic file system and thus limited the scope of the problem to the superblock, it is time to fix the problem. You can do this by copying the backup superblock back the the location of the old superblock. You can do that using
dd if=/dev/hda5 of=/dev/hda5 bs=1024 skip=131072 count=1 seek=1.
Once you've finished, your file system is accessible again in the way it was before.
Problems mounting ext3 file systems often occur because of a problem in the administrative information at the beginning of the file system. If the problem comes from the superblock, you can mount the file system anyway, using a backup superblock and the mount option -o sb. Using dd, you can even restore the superblock to its original location.
This was first published in March 2007