270 The File System
might be useful. Few people, that is, except for Macintosh users, who have
known and enjoyed file types since 1984.No Record Lengths
Despite the number of databases stored on Unix systems, the Unix file sys-
tem, by design, has no provision for storing a record length with a file.
Again, storing and maintaining record lengths is left to the programmer.
What if you get it wrong? Again, this depends on the program that you're
using. Some programs will notice the difference. Most won’t. This means
that you can have one program that stores a file with 100-byte records, and
you can read it back with a program that expects 200-byte records, and
won’t know the difference. Maybe...All of Unix’s own internal databases—the password file, the group file, the
mail aliases file—are stored as text files. Typically, these files must be pro-
cessed from beginning to end whenever they are accessed. “Records”
become lines that are terminated with line-feed characters. Although this
method is adequate when each database typically had less than 20 or 30
lines, when Unix moved out into the “real world” people started trying to
put hundreds or thousands of entries into these files. The result? Instant
bottleneck trying to read system databases. We’re talking real slowdown
here. Doubling the number of users halves performance. A real system
wouldn’t be bothered by the addition of new users. No less than four mutu-
ally incompatiable workarounds have now been developed to cache the
information in /etc/password, /etc/group, and other critical databases. All
have their failings. This is why you need a fast computer to run Unix.File and Record Locking
“Record locking” is not a way to keep the IRS away from your financial
records, but a technique for keeping them away during the moments that
you are cooking them. The IRS is only allowed to see clean snapshots, lest
they figure out what you are really up to. Computers are like this, too. Two
or more users want access to the same records, but each wants private
access while the others are kept at bay. Although Unix lacks direct record
support, it does have provisions for record locking. Indeed, many people
are surprised that modern Unix has not one, not two, but three completely
different systems for record locking.In the early days, Unix didn’t have any record locking at all. Locking vio-
lated the “live free and die” spirit of this conceptionally clean operating
system. Ritchie thought that record locking wasn't something that an oper-
ating system should enforce—it was up to user programs. So when Unix