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Appendix B: F
sck
-The U
nix
File System Check Program
1
B.1 Introduction
When a U
nix
operating system is brought up, a consistency check of the file
systems should always be performed. This precautionary measure helps to
insure a reliable environment for file storage on disk. If an inconsistency is
discovered, corrective action must be taken. No changes are made to any file
system by fsck without prior operator approval.
The purpose of this memo is to dispel the mystique surrounding file system
inconsistencies. It first describes the updating of the file system (the calm
before the storm) and then describes file system corruption (the storm).
Finally, the set of heuristically sound corrective actions used by fsck (the Coast
Guard to the rescue) is presented.
B.2 Update of the File System
Every working day hundreds of files are created, modified, and removed.
Every time a file is modified, the U
nix
operating system performs a series of
file system updates. These updates, when written on disk, yield a consistent
file system. To understand what happens in the event of a permanent
interruption in this sequence, it is important to understand the order in which
the update requests were probably being honored. Knowing which pieces of
information were probably written to the file system first, heuristic procedures
can be developed to repair a corrupted file system.
There are five types of file system updates. These involve the super-block,
inodes, indirect blocks, data blocks (directories and files), and free-list blocks.
Super-Block
The super-block contains information about the size of the file system, the size
of the inode list, part of the free-block list, the count of free blocks, the count
1. This appendix is modified from a paper with the same name by T. J. Kowalski.
Version 1.0