UPDATE(8) System Manager's Manual UPDATE(8)
update - periodically update the super block
Update is a program that executes the sync(2) primitive every 30 sec-
onds. This insures that the file system is fairly up to date in case
of a crash. This command should not be executed directly, but should
be executed out of the initialization shell command file.
sync(2), sync(1), init(8)
With update running, if the CPU is halted just as the sync is executed,
a file system can be damaged. This is partially due to DEC hardware
that writes zeros when NPR requests fail. A fix would be to have
sync(1) temporarily increment the system time by at least 30 seconds to
trigger the execution of update. This would give 30 seconds grace to
halt the CPU.