This section describes the kinds of consistency checks that the fsck command applies to these UFS file system components: superblock, cylinder group blocks, inodes, indirect blocks, and data blocks.
For information about UFS file system structures, see Structure of Cylinder Groups for UFS File Systems.
The superblock stores summary information, which is the most commonly corrupted component in a UFS file system. Each change to 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. 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 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 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.
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.
The list of inodes is checked sequentially starting with inode 2. (Inode 0 and inode 1 are reserved). Each inode is checked for inconsistencies in the following:
Format and type
Bad block numbers
Shadow (used for ACLs)
When the file system is created, a fixed number of inodes are set aside. However, these inodes are not allocated until they are needed. An allocated inode is one that points to a file. An unallocated inode does not point to a file and, therefore, should be empty. The partially allocated state means that the inode is incorrectly formatted. An inode can get into this state if, for example, bad data is written into the inode list because of a hardware failure. The only corrective action the fsck command can take is to clear the inode.
Each inode contains a count of the number of directory entries linked to it. The fsck command verifies the link count of each inode by examining the entire directory structure, starting from the root (/) directory, and calculating an actual link count for each inode.
Discrepancies between the link count stored in the inode and the actual link count as determined by the fsck command might be one of three types:
The stored count is not 0, and the actual count is 0.
This condition can occur if no directory entry exists for the inode. In this case, the fsck command puts the disconnected file in the lost+found directory.
The stored count is not 0 and the actual count is not 0. However, the counts are unequal.
This condition can occur if a directory entry has been added or removed, but the inode has not been updated. In this case, the fsck command replaces the stored link count with the actual link count.
The stored count is 0, and the actual count is not 0.
In this case, the fsck command changes the link count of the inode to the actual count.
Each inode contains a list, or pointers to lists (indirect blocks), of all the blocks claimed by the inode. Because indirect blocks are owned by an inode, inconsistencies in indirect blocks directly affect the inode that owns the indirect block.
The fsck command compares each block number claimed by an inode to a list of allocated blocks. If another inode already claims a block number, the block number is put on a list of duplicate blocks. Otherwise, the list of allocated blocks is updated to include the block number.
If duplicate blocks are found, the fsck command makes a second pass of the inode list to find the other inode that claims each duplicate block. The fsck command cannot determine with certainty which inode is in error. So, the fsck command prompts you to choose which inode should be kept and which inode should be cleared. Note that a large number of duplicate blocks in an inode might be caused by an indirect block not being written to the file system
The fsck command checks each block number claimed by an inode to determine whether its value is higher than the value of the first data block and lower than that of the last data block in the file system. If the block number is outside this range, it is considered a bad block number.
Bad block numbers in an inode might be caused by an indirect block not being written to the file system. The fsck command prompts you to clear the inode.
Each inode contains a count of the number of data blocks that it references. The number of actual data blocks is the sum of the allocated data blocks and the indirect blocks. The fsck command computes the number of data blocks and compares that block count against the number of blocks that the inode claims. If an inode contains an incorrect count, the fsck command prompts you to fix it.
Each inode contains a 64-bit size field. This field shows the number of characters (data bytes) in the file associated with the inode. A rough check of the consistency of the size field of an inode uses the number of characters shown in the size field to calculate how many blocks should be associated with the inode, and then compares that number to the actual number of blocks claimed by the inode.
Blocks already claimed by another inode
Block numbers outside the range of the file system
These consistency checks listed are also performed for direct blocks.
Plain data blocks
Symbolic-link data blocks
Directory data blocks
Plain data blocks contain the information stored in a file. Symbolic-link data blocks contain the path name stored in a symbolic link. Directory data blocks contain directory entries. The fsck command can check only the validity of directory data blocks.
Directories are distinguished from regular files by an entry in the mode field of the inode. Data blocks associated with a directory contain the directory entries. Directory data blocks are checked for inconsistencies involving the following:
Directory inode numbers that point to unallocated inodes
Directory inode numbers that are greater than the number of inodes in the file system
Incorrect directory inode numbers for “.” and “..” directories
Directories that are disconnected from the file system
If the inode number in a directory data block points to an unallocated inode, the fsck command removes the directory entry. This condition can occur if the data blocks that contain a new directory entry are modified and written out, but the inode does not get written out. This condition can occur if the CPU is shut down abruptly.
The directory inode number entry for “.” must be the first entry in the directory data block. The directory inode number must reference itself. That is, its value must be equal to the inode number for the directory data block.
The directory inode number entry for “..” must be the second entry in the directory data block. The directory inode number value must be equal to the inode number of the parent directory or the inode number of itself if the directory is the root (/) directory).
If the directory inode numbers for “.” and “..” are incorrect, the fsck command replaces them with the correct values. If there are multiple hard links to a directory, the first hard link found is considered the real parent to which “..” should point. In this case, the fsck command recommends that you have it delete the other names.
The fsck command checks the general connectivity of the file system. If a directory that is not linked to the file system is found, the fsck command links the directory to the lost+found directory of the file system. This condition can occur when inodes are written to the file system. However, the corresponding directory data blocks are not.