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. fsck can check the validity only 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:
Directory inode numbers pointing to unallocated inodes
Directory inode numbers greater than the number of inodes in the file system
Incorrect directory inode numbers for "." and ".." directories
Directories disconnected from the file system
If the inode number in a directory data block points to an unallocated inode, fsck removes the directory entry. This condition can occur if the data blocks containing 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 halted without warning.
The directory inode number entry for "." must be the first entry in the directory data block. It 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. Its 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, fsck replaces them with the correct values. If there are multiple hard links to a directory, the first one found is considered the real parent to which ".." should point. In this case, fsck recommends you have it delete the other names.
The fsck program checks the general connectivity of the file system. If a directory is found that is not linked to the file system, fsck links the directory to the lost+found directory of the file system. (This condition can occur when inodes are written to the file system but the corresponding directory data blocks are not.)