UNIX Filesystems: Evolution, Design, and Implementation

Repairing Damaged Filesystems

A filesystem can typically be in one of two states, either clean or dirty. To mount a filesystem it must be clean, which means that it is structurally intact. When filesystems are mounted read/write, they are marked dirty to indicate that there is activity on the filesystem. Operations may be pending on the filesystem during a system crash, which could leave the filesystem with structural damage. In this case it can be dangerous to mount the filesystem without knowing the extent of the damage. Thus, to return the filesystem to a clean state, a filesystem-specific check program called fsck must be run to repair any damage that might exist.

For example, consider the following call to mount after a system crash:

# mount -F vxfs /dev/vx/dsk/vol1 /mnt1
UX:vxfs mount: ERROR: /dev/vx/dsk/vol1 is corrupted, needs checking

The filesystem is marked dirty and therefore the mount fails. Before it can be mounted again, the VxFS fsck program must be run as follows:

# fsck -F vxfs /dev/vx/rdsk/vol1
log replay in progress
replay complete marking super-block as CLEAN

VxFS is a transaction-based filesystem in which structural changes made to the filesystem are first written to the filesystem log. By replaying the transactions in the log, the filesystem returns to its clean state.

Most UNIX filesystems are not transaction-based, and therefore the whole filesystem must be checked for consistency. In the example below, a full fsck is performed on a UFS filesystem to ...