pwolfe@kailand.kai.com (09/07/89)
I was just re-reading the 1988 SURF Proceedings section from Ron Parsons'
"System Tuning Workshop" and found a minor mistake that might affect some
people.
He mentions "... another thing to do is watch df. ... You should always keep
the amount of disk consumption below 90 percent for all partitions. The
Berkeley File system gets rather slow if you fill the disk all the way to its
capacity".
While it is true that the Berkeley file system shows severe performance
degredation when less than 10% free space is maintained (as noted in the
article "A Fast File System for UNIX", found in the Dynix Programmers Manual
VOL II part 2), it is also true that the "newfs(8)" program automatically
reserves 10% of each partition when initializing it. The "capacity" column
that df displays does not include that reserved 10%.
This is also the explanation for frequently asked question "why doesn't the df
command's "avail" plus "used" equal the total "kbytes"?". The answer is that
the total kbytes column includes that 10% of the partition that (non-root)
users cannot make use of.
So, my point is, system administrators do not need to make an effort to keep
disk partitions below 90% for performance reasons. DYNIX does that already.
If "df" says the partition is at or over 100% full, then users are not allowed
to write on it anyway (but root can - once in a while /usr gets 102% full).
Patrick Wolfe (pat@kai.com, {uunet,uiucuxc,sequent}!kailand!pat)
System Manager, Kuck & Associates, Inc.