System Administration Guide, Volume 1

How the File System State Is Recorded

The 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 /sbin/rcS 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 flag values are described in the table below.

Table 39-1 State Flag Values


State Flag Value	Description
`FSACTIVE`	When a file system is mounted and then modified, the state flag is set to `FSACTIVE`. The file system might 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 might be inconsistent.
`FSBAD`	If 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.
`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.
`FSLOG`	If the file system was mounted with UFS logging, the state flag is set to `FSLOG`. Any file system with an `FSLOG` 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 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 might lose a small amount of data. File systems that are marked `FSSTABLE` can skip the checking before mounting. The mount(2) system call will not mount a file system for read/write if the file system state is not `FSCLEAN`, `FSSTABLE`, or `FSLOG`.

The table below shows how the state flag is modified by the fsck command, based on its initial state.

Table 39-2 How the State Flag is Modified by fsck


Inital State: Before `fsck`		State After `fsck`
	No Errors	All Errors Corrected	Uncorrected Errors
unknown	`FSSTABLE`	`FSSTABLE`	unknown
`FSACTIVE`	`FSSTABLE`	`FSSTABLE`	`FSACTIVE`
`FSSTABLE`	`FSSTABLE`	`FSSTABLE`	`FSACTIVE`
`FSCLEAN`	`FSCLEAN`	`FSSTABLE`	`FSACTIVE`
`FSBAD`	`FSSTABLE`	`FSSTABLE`	`FSBAD`
`FSLOG`	`FSLOG`	`FSLOG`	`FSLOG`