The superblock stores summary information, which is the most commonly corrupted component in a UFS file system. Each change to the file system inodes or data blocks also modifies the superblock. If the CPU is halted and the last command is not a sync command, the superblock almost certainly becomes corrupted.
The superblock is checked for inconsistencies in the following:
File system size
Number of inodes
Free block count
Free inode count
The file system size must be larger than the number of blocks used by the superblock and the list of inodes. The number of inodes must be less than the maximum number allowed for the file system. An inode represents all the information about a file. The file system size and layout information are the most critical pieces of information for the fsck command. Although there is no way to actually check these sizes because they are statically determined when the file system is created. However, the fsck command can check that the sizes are within reasonable bounds. All other file system checks require that these sizes be correct. If the fsck command detects corruption in the static parameters of the primary superblock, it requests the operator to specify the location of an alternate superblock.
For more information about the structure of the UFS file system, see The Structure of Cylinder Groups for UFS File Systems.
Free blocks are stored in the cylinder group block maps. The fsck command checks that all the blocks marked as free are not claimed by any files. When all the blocks have been accounted for, the fsck command checks to see if the number of free blocks plus the number of blocks that are claimed by the inodes equal the total number of blocks in the file system. If anything is wrong with the block maps, the fsck command rebuilds them, leaving out blocks already allocated.
The summary information in the superblock includes a count of the total number of free blocks within the file system. The fsck command compares this count to the number of free blocks it finds within the file system. If the counts do not agree, the fsck command replaces the count in the superblock with the actual free-block count.
The summary information in the superblock contains a count of the free inodes within the file system. The fsck command compares this count to the number of free inodes it finds within the file system. If the counts do not agree, fsck replaces the count in the superblock with the actual free inode count.