UNIX 4.2 thrashing - the cause?

[Rick Lindsley]

I don't think you understand...the file systems, in the example you
gave, ARE only 90% full. If you add the free and used, it does not
equal (the partition - superblock). 4.2 has ALREADY reserved that 10%
for you, and if you have a relatively inactive file system which you
are just using for convenient storage, and efficiency is not as big a
concern as available space, it may be worth your while to regain that
space for yourself.  Check out tunefs(8).

This is not to say your solution is incorrect -- anytime a disk is
near-full you will suffer a slight but increasing degradation as files
fragment hither and yon. It IS better to spread things around. But your
view of the 10% threshold is not entirely correct.

Rick Lindsley