How to Restore a Bad Superblock

1. Become superuser.

2. Change to a directory outside the damaged file system.

3. Unmount the file system.

   # umount mount-point

   ---

   Caution -

   Be sure to use the newfs -N in the next step. If you omit the -N option, you will create a new, empty file system.

   ---

4. Display the superblock values with the newfs -N command.

   # newfs -N /dev/rdsk/device-name

   The output of this command displays the block numbers that were used for the superblock copies when newfs created the file system, unless the file system was created with special parameters. See "Deciding on Custom File System Parameters" for information on creating a customized file system.

5. Provide an alternative superblock with the fsck command.

   # fsck -F ufs -o b=block-number /dev/rdsk/device-name

   fsck uses the alternative superblock you specify to restore the primary superblock. You can always try 32 as an alternative block, or use any of the alternative blocks shown by newfs -N.

Example--Restoring a Bad Superblock

The following example restores the superblock copy 5264 for the /files7 file system:

# cd /
# umount /files7
# newfs -N /dev/rdsk/c0t3d0s7
/dev/rdsk/c0t3d0s7: 163944 sectors in 506 cylinders of 9 tracks, 36 sectors
 83.9MB in 32 cyl groups (16 c/g, 2.65MB/g, 1216 i/g)
super-block backups (for fsck -b #) at:
 32, 5264, 10496, 15728, 20960, 26192, 31424, 36656, 41888,
 47120, 52352, 57584, 62816, 68048, 73280, 78512, 82976, 88208,
 93440, 98672, 103904, 109136, 114368, 119600, 124832, 130064, 135296,
 140528, 145760, 150992, 156224, 161456,
# fsck -F ufs -o b=5264 /dev/rdsk/c0t3d0s7
Alternate superblock location: 5264.
** /dev/rdsk/c0t3d0s7
** Last Mounted on
** Phase 1 - Check Blocks and Sizes
** Phase 2 - Check Pathnames
** Phase 3 - Check Connectivity
** Phase 4 - Check Reference Counts
** Phase 5 - Check Cyl groups
36 files, 867 used, 75712 free (16 frags, 9462 blocks, 0.0% fragmentation)
/dev/rdsk/c0t3d0s7 FILE SYSTEM STATE SET TO OKAY
 
***** FILE SYSTEM WAS MODIFIED *****
# 

If the superblock in the root (/) file system becomes damaged and you cannot boot the system, reinstall /kernel/unix and rebuild the root (/) file system with newfs. Because a superblock is created by the newfs command, you do not need to restore it.