Swapon (4.{2,3})

[dem at uwslh.UUCP]

Recently Mike Williams asked about swapping performance with several
disks of varying speed.  We tried this experiment once.
We had an eagle and a CDC9730-82 (equivalent to a 67 Meg RM0something).
The CDC drive had an access time about 50% longer than the eagle.
We had been swapping just on the eagle, but the literature (and the
kernel (4.2 BSD) every time we booted) said we should split the swap
space over multiple devices.
We split it between the eagle and the CDC drive and the performance
became dramatically WORSE.  We shut down, rebooted and took out the
split swapping within several hours because it was so bad.
>From this I concluded that splitting your swap space is beneficial
only if your disks are of similar speed, otherwise you should just
swap on the fastest one.
I found this particulary suprising because we are running a VAX 750
with 8 Meg of memory and we almost never have more than 4 Meg in use.
I would be interested in any explanation from the net of:
1.  Why did performance get so much worse?
2.  Under what circumstances does splitting the swap space help.
    Does it matter if you have too little memory or more than enough?
-- 
David E. Miran         ...!{seismo,harvard,topaz,ihnp4}!uwvax!uwslh!dem
Wisconsin State Hygiene Lab    or    uwslh!dem at rsch.wisc.edu
University of Wisconsin          (608) 262-0019
465 Henry Mall
Madison, WI  53706