262 The File System
What’s a File System?
A file system is the part of a computer’s operating system that manages file
storage on mass-storage devices such as floppy disks and hard drives. Each
piece of information has a name, called the filename, and a unique place
(we hope) on the hard disk. The file system’s duty is to translate names
such as /etc/passwd into locations on the disk such as “block 32156 of hard
disk #2.”' It also supports the reading and writing of a file’s blocks.
Although conceptually a separable part of the operating system, in practice,
nearly every operating system in use today comes with its own peculiar file
system.Meet the Relatives
In the past two decades, the evil stepmother Unix has spawned not one, not
two, but four different file systems. These step-systems all behave slightly
differently when running the same program under the same circumstances.The seminal Unix File System (UFS), the eldest half-sister, was sired in
the early 1970s by the original Unix team at Bell Labs. Its most salient fea-
ture was its freewheeling conventions for filenames: it imposed no restric-
tions on the characters in a filename other than disallowing the slash
character (“/”) and the ASCII NUL. As a result, filenames could contain a
multitude of unprintable and (and untypable) characters, a “feature” often
exploited for its applications to “security.” Oh, UFS also limited filenames
to 14 characters in length.The Berkeley Fast (and loose) File System (FFS) was a genetic make-
over of UFS engineered at the University of California at Berkeley. It
wasn’t fast, but it was faster than the UFS it replaced, much in the same
way that a turtle is faster than a slug.Berkeley actually made a variety of legitimate, practical improvements to
the UFS. Most importantly, FFS eliminated UFS’s infamous 14-character
filename limit. It introduced a variety of new and incompatible features.
Foremost among these was symbolic links—entries in the file system that
could point to other files, directories, devices, or whatnot. Nevertheless,
Berkeley’s “fixes” would have been great had they been back-propagated
to Bell Labs. But in a classic example of Not Invented Here, AT&T refused
Berkeley’s new code, leading to two increasingly divergent file systems
with a whole host of mutually incompatiable file semantics. Throughout
the 1980s, some “standard” Unix programs knew that filenames could be
longer than 14 characters, others didn’t. Some knew that a “file” in the file