How to increase NFS Performance?

[Per Foreby]

What is the best thing I can do in order to increase NFS performance on
our central file server?

Facts:
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* Sun 3/280
* SunOs 4.1
* 8 Mb memory
* 5 Gb disk (6 disks on 2 xd753)
* Serves approx 100 clients with USER files (the clients have their
  root and swap on other servers)  
* Low write percentage (about 3%) (write + create + remove)
* CPU idle time > 50%
* Load average approx 5.

Possible solutions:
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1. Upgrade to sun4 (4/380?)
   * Does this help when the cpu is idle most of the time?

2. Add more memory
   * Gives me more diskcache. 
   * Is there any way to list some stats for the cache? vmstat showes about
     250 K free list size, but does this include the disk cache, or can the
     free list borrow pages from the disk cache if it needs more?

3. Buy a Prestoserve
   * Is this really usefull when the write percentage is low?

Please email, I'll summarize.

Per Foreby; perf at efd.lth.se; LTH-E, Box 118, S-221 00 LUND, Sweden; 046-107492