14.6. Journaling File Systems

The ext2 file system is a good example of a traditional UNIX file system, and suffers from a classic limitation of such file systems: after a system crash, a file-system consistency check (fsck) must be performed on reboot in order to ensure the integrity of the file system. This is necessary because, at the time of the system crash, a file update may have been only partially completed, and the file-system metadata (directory entries, i-node information, and file data block pointers) may be in an inconsistent state, so that the file system might be further damaged if these inconsistencies are not repaired. A file-system consistency check ensures the consistency of the file-system metadata. Where possible, repairs are performed; otherwise, information that is not retrievable (possibly including file data) is discarded.

The problem is that a consistency check requires examining the entire file system. On a small file system, this may take anything from several seconds to a few minutes. On a large file system, this may require several hours, which is a serious problem for systems that must maintain high availability (e.g., network servers).