The UFS file system relies on an internal set of tables to keep track of inodes used and available blocks. When these internal tables are not properly synchronized with data on a disk, inconsistencies result and file systems need to be repaired.
File systems can be inconsistent because of abrupt termination of the operating system in these ways:
Accidental unplugging of the system
Turning off the system without proper shutdown procedure
A software error in the kernel
File system inconsistencies, while serious, are not common. When a system is booted, a check for file system consistency is automatically performed (with the fsck command). Most of the time, this file system check repairs problems it encounters.
The fsck command places files and directories that are allocated but unreferenced in the lost+found directory. A inode number is assigned as the name of unreferenced file and directory. If the lost+found directory does not exist, the fsck command creates it. If there is not enough space in the lost+found directory, the fsck command increases its size.
For a description of inodes, see Inodes.