ugh.book

266 The File System

while the update is taking places. (Interrupting these sorts of operations can

be like interrupting John McEnroe during a serve: both yield startling and

unpredictable results.)

No matter! The people who designed the Unix File System didn’t think that

the computer would crash very often. Rather than taking the time to design

UFS so that it would run fast and keep the disk consistent (it is possible to

do this), they designed it simply to run fast. As a result, the hard disk is

usually in an inconsistent state. As long as you don’t crash during one of

these moments, you’re fine. Orderly Unix shutdowns cause no problems.

What about power failures and glitches? What about goonball technicians

and other incompetent people unplugging the wrong server in the machine

room? What about floods in the sewers of Chicago? Well, you’re left with

a wet pile of noodles where your file system used to be. The tool that tries

to rebuild your file system from those wet noodles is fsck (pronounced “F-

sick,”) the file system consistency checker. It scans the entire file system

looking for damage that a crashing Unix typically exacts on its disk. Usu-

ally fsck can recover the damage. Sometimes it can’t. (If you’ve been hav-

ing intermittent hardware failures, SCSI termination problems, and

incomplete block transfers, frequently it can’t.) In any event, fsck can take

5, 10, or 20 minutes to find out. During this time, Unix is literally holding

your computer hostage.

Here’s a message that was forwarded to UNIX-HATERS by MLY; it orig-

inally appeared on the Usenet Newsgroup comp.arch in July 1990:

Date: 13 Jul 9016:58:55 GMT

From: [email protected] (Andy Glew)^2

Subject: Fast Re-booting

Newsgroups: comp.arch

A few years ago a customer gave us a <30 second boot after power

cycle requirement, for a real-time OS. They wanted <10.

This DECstation 3100, with 16MB of memory, and an approxi-

mately 300Mb local SCSI disk, took 8:19 (eight minutes and nine-

teen seconds) to reboot after powercycle. That included fsck’ing the

disk. Time measured from the time I flicked the switch to the time I

could log in.

(^2) Forwarded to UNIX-HATERS by Richard Mlynarik.