Restoring a Bad Superblock

When the superblock of a file system becomes damaged, you must restore it. The fsck command tells you when a superblock is bad. Fortunately, copies of the superblock are stored within a file system.

You can use the fsck -o b command to replace the superblock with one of these copies or use fsck's automatic search for backup superblocks feature, which is new in the Solaris Express release. For more information about this feature, see Automatic Search for Backup Superblocks.

For more information about the superblock, see Superblock.

If the superblock in the root (/) file system becomes damaged and you cannot restore it, you have two choices:

Reinstall the system.
Boot from the network or local CD, and attempt the following steps. If these steps fail, recreate the root (/) file system by using the newfs command and restore it from a backup copy.

How to Restore a Bad Superblock ( Solaris Express Release)

This procedure is new in the Solaris Express release. If your file system has a bad superblock, fsck automatically calculates an alternative superblock as seen in the following messages:

BAD SUPERBLOCK AT ...

LOOK FOR ALTERNATE SUPERBLOCKS WITH MKFS? 
LOOK FOR ALTERNATE SUPERBLOCKS WITH NEWFS?

Caution - If a file system with a damaged superblock was created with newfs or mkfs customized parameters, such as ntrack or nsect, using fsck's automatically calculated superblock for the repair process could irreparably damage your file system.

In the case of a file system that was created with customized parameters and it has a bad superblock, fsck provides the following prompt to cancel the fsck session:

CANCEL FILESYSTEM CHECK?

Canceling the fsck session would be an appropriate response if this file system was created with customized parameters or if there is some other concern about running fsck on this file system.

Become superuser or assume an equivalent role.

Check the file system with the suspected bad superblock.

# fsck /dev/rdsk/c0t1d0s0

** /dev/rdsk/c0t1d0s0

BAD SUPERBLOCK at ...

Determine how the file system was created and select one of the following:

The file system was created with the newfs command.

fsck responds that all superblocks are corrupt and it must use a generic superblock. Answer the fsck prompts as described in the example below.

Caution - Do not use this option if the file system was created with customized parameters. This option should only be used as a last resort. Be prepared to restore the file system from a backup copy.

# fsck /dev/dsk/c1t2d0s0
** /dev/rdsk/c1t2d0s0
BAD SUPERBLOCK AT BLOCK 16: BLOCK SIZE LARGER THAN MAXIMUM SUPPORTED

LOOK FOR ALTERNATE SUPERBLOCKS WITH MKFS? no


LOOK FOR ALTERNATE SUPERBLOCKS WITH NEWFS? yes

SEARCH FOR ALTERNATE SUPERBLOCKS FAILED.

USE GENERIC SUPERBLOCK FROM MKFS? no


USE GENERIC SUPERBLOCK FROM NEWFS? yes

CALCULATED GENERIC SUPERBLOCK WITH NEWFS
If filesystem was created with manually-specified geometry, using
auto-discovered superblock may result in irrecoverable damage to
filesystem and user data.

CANCEL FILESYSTEM CHECK? no

** Last Mounted on
** Phase 1 - Check Blocks and Sizes
** Phase 2 - Check Pathnames
** Phase 3a - Check Connectivity
** Phase 3b - Verify Shadows/ACLs
** Phase 4 - Check Reference Counts
** Phase 5 - Check Cylinder Groups
CORRECT GLOBAL SUMMARY
SALVAGE? y


UPDATE STANDARD SUPERBLOCK? y

81 files, 3609 used, 244678 free (6 frags, 30584 blocks, 0.0% fragmentation)

***** FILE SYSTEM WAS MODIFIED *****

fsck responds that it found an alternate superblock with a message similar to the following:
```
FOUND ALTERNATE SUPERBLOCK 32 WITH NEWFS
```
With this fsck scenario, follow the prompts as shown in Automatic Search for Backup Superblocks.

The file system was created with the mkfs command.

fsck responds that all superblocks are corrupt and must use a generic superblock. Answer the fsck prompts as described in the example below.

# fsck /dev/dsk/c1t2d0s0
** /dev/rdsk/c1t2d0s0
BAD SUPERBLOCK AT BLOCK 16: BLOCK SIZE LARGER THAN MAXIMUM SUPPORTED

LOOK FOR ALTERNATE SUPERBLOCKS WITH MKFS? yes


LOOK FOR ALTERNATE SUPERBLOCKS WITH NEWFS? no

SEARCH FOR ALTERNATE SUPERBLOCKS FAILED.

USE GENERIC SUPERBLOCK FROM MKFS? yes

CALCULATED GENERIC SUPERBLOCK WITH MKFS
If filesystem was created with manually-specified geometry, using
auto-discovered superblock may result in irrecoverable damage to
filesystem and user data.

CANCEL FILESYSTEM CHECK? no

** Last Mounted on
** Phase 1 - Check Blocks and Sizes
** Phase 2 - Check Pathnames
** Phase 3a - Check Connectivity
** Phase 3b - Verify Shadows/ACLs
** Phase 4 - Check Reference Counts
** Phase 5 - Check Cylinder Groups
CORRECT GLOBAL SUMMARY
SALVAGE? y


UPDATE STANDARD SUPERBLOCK? y

81 files, 3609 used, 243605 free (117 frags, 30436 blocks, 0.0% fragmentation)

fsck responds that it found an alternate superblock with a message similar to the following:
```
FOUND ALTERNATE SUPERBLOCK 32 WITH MKFS
```
With this fsck scenario, follow the prompts as shown in Automatic Search for Backup Superblocks.

Answer the prompts to salvage and restore the superblock.
There is no need to rerun fsck when you see the following message:
```
***** FILE SYSTEM WAS MODIFIED *****
```
However, it doesn't harm the file system to rerun fsck after this message. This message is just informational about fsck's corrective actions.

How to Restore a Bad Superblock (Solaris 8, 9, and 10 Releases)

Become superuser or assume an equivalent role.
Determine whether the bad superblock is in the root (/), /usr, or /var file system and select one of the following:
- If the bad superblock is in either the root (/), /usr, or /var file system, then boot from the network or a locally connected CD.
  From a locally-connected CD, use the following command:
```
ok boot cdrom -s
```
  From the network where a boot or install server is already setup, use the following command:
```
ok boot net -s
```
  If you need help stopping the system, see Chapter 10, Booting a System (Tasks), in System Administration Guide: Basic Administration or Chapter 12, Booting a Solaris System With GRUB (Tasks), in System Administration Guide: Basic Administration.
- If the bad superblock is not in either the root (/), /usr, /var file system, change to a directory outside the damaged file system and 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 destroy all of the data in the file system and replace it with an empty file system.
Display the superblock values by using the newfs -N command.
```
# newfs -N /dev/rdsk/device-name
```
The command output displays the block numbers that were used for the superblock copies when the newfs command created the file system, unless the file system was created with special parameters. For information on creating a customized file system, see Customizing UFS File System Parameters.
Provide an alternate superblock by using the fsck command.
```
# fsck -F ufs -o b=block-number /dev/rdsk/device-name
```
The fsck command uses the alternate superblock you specify to restore the primary superblock. You can always try 32 as an alternate block. Or, use any of the alternate blocks shown by the newfs -N command.

Example 22-3 Restoring a Bad Superblock (Solaris 8, 9, and 10 Releases)

The following example shows how to restore the superblock copy 5264.

# 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)

***** FILE SYSTEM WAS MODIFIED *****
#