System Administration Guide: Basic Administration

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 a file system needs to be checked for consistency. The flag is used by the /sbin/rcS script during booting and by the fsck -m command. If you ignore the result from the fsck -m command, all file systems can be checked regardless of the setting of the state flag.

For a description of the superblock, see The Superblock.

The possible state flag values are described in the following table.

Table 43–1 Values of File System State Flags


State Flag Values	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 is 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 the `fsck` command 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 the `fsck` command on the raw root device. Then remount the root (`/`) file system with read and write access.
`FSCLEAN`	If a file system is 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 a file system is 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`) that 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 as `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 as `FSSTABLE` can skip the checking before mounting. The `mount` command will not mount a file system for read and write access if the file system state is not `FSCLEAN`, `FSSTABLE`, or `FSLOG`.

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

Table 43–2 How the State Flag is Modified by fsck


Initial 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`