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

VTOC Label은 1TB 이상의 Disk를 지원하지 못하는 한계점을 가지고 있다. 1TB 이상의 Disk를 사용하기 위해서 EFI (Extensible Firmware Unterface) Disk Label을 사용할 수 있다. 1TB 이하의 Disk에서 EFI Label을 사용하고자 할 경우에는 format -e 명령어를 사용할 수 있다.
 
EFI Label과 VTOC Label의 비교
* EFI Label은 1TB 이상의 Disk를 지원한다.
* 0-6 번의 slice를 사용할 수 있으며, 2번 slice도 하나의 slice로 사용한다.
* EFI Label의 크기는 보통 34 sector이다.

EFI Lable의 한계점
* SCSI driver, ssd에 대해서 현제 2TB까지만 사용 가능하다. 2TB 이상의 Disk를 사용하고자 할 경우 Volume Manager를 사용하여햐 한다.
* Solaris 9 4/03 버전부터 사용 가능
* IDE Disk는 지원하지 않는다.
* EFI Label을 사용하는 disk는 boot disk로 사용할 수 없다.
* format 명령어에서 save option을 사용할 수 없다.

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:: VTOC (Volume Table of Contents)
VTOC has the information of disk's controller, geometry and slices(partitions).
The first sector of disk has VTOC.

:: format
Format command loads VTOC on memory, so you can modify it.
By modifying VTOC you can create or remove partitions.
You have to excute 'label' after you finish modifying VTOC,
so the new VTOC can be saved and effective.

:: Backing up VTOC
1. On the format command, excute 'save' to back up VTOC.
The back up file is /etc/format.dat(recommanded path).
To restore VTOC, excute 'select'.
2. Use 'prtvtoc' command for direct VTOC backup.
(ex : #prtvtoc /dev/rdisk/c#t#d# > /vtoc/c#t#d#)
You can use 'fmthard' for VTOC backup.