File systems can be damaged or become inconsistent because of abrupt termination of the operating system in these ways:
- Power failure
- Accidental unplugging of the system
- Turning the system off without proper shutdown procedure
- A software error in the kernel

When a system is booted, a file system consistency check is automatically done. Most of the time, this file system check repairs problems it encounters. File systems are checked with the fsck (file system check) program.

* Understanding How the File System State Is Recorded
fsck command uses a state flag, which is stored in the superblock, to record the condition of the file system. This flag is used by the fsck command to determine whether or not a file system needs to be checked for consistency. The flag is used by the /etc/bcheckrc script during booting and by the fsck command when run from a command line using the -m option. If you ignore the result from the -m option to fsck, all file systems can be checked regardless of the setting of the state flag.

The possible state values are:
* FSCLEAN - If the file system was unmounted properly, the state flag is set to FSCLEAN. Any file system with an FSCLEAN state flag is not checked when the system is booted.
* FSSTABLE - The file system is (or was) mounted but has not changed since the last checkpoint: sync or fsflush--which normally occurs every 30 seconds. For example, the kernel periodically checks to see if a file system is idle and, if so, flushes the information in the superblock back to the disk and marks it FSSTABLE. If the system crashes, the file system structure is stable, but users may lose a small amount of data. File systems that are marked FSSTABLE can skip the checking before mounting.
* FSACTIVE - When a file system is mounted and then modified, the state flag is set to FSACTIVE. The file system may contain inconsistencies. A file system will be marked as FSACTIVE before any modified metadata is written to the disk. When a file system is unmounted gracefully, the state flag is set to FSCLEAN. A file system with the FSACTIVE flag must be checked by fsck because it may be inconsistent. The mount(2) system call will not mount a file system for read/write if the file system state is not FSCLEAN or FSSTABLE.
* FSBAD - When the root file system is mounted when its state is not FSCLEAN or FSSTABLE, the state flag is set to FSBAD. The kernel will not change this file system state to FSCLEAN or FSSTABLE. If a root file system is flagged FSBAD as part of the boot process, it will be mounted read-only. You can run fsck on the raw root device. Then remount the root file system as read/write.

State Flag Transitions After fsck

Before fsck	After fsck
Initial State	No Errors	New State All Errors Corrected	Uncorrected Errors
unknown	stable	stable	unknown
active	stable	stable	active
stable	stable	stable	active
clean	clean	stable	active
bad	stable	stable	bad

from http://docs.sun.com/app/docs/doc/801-6631/6i10bkb0u?l=ko&a=view