It is true that one can add application server hosts in an attempt to
improve performance. However, in an RDBMS-based application, tuning disk
access is very important. You can have many CPUs but if you have one disk
with no cache in front of it, your throughput will be limited by disk seek
time.

There are several companies which make file servers with large,
non-volatile, write-back caches and many RAID-ed disks: www.netapp.com,
www.emc.com, www.seek.com.

It is also important to profile your app because sometimes one is surprised
where the time is going. Examples of silly things I've encountered:
* Servlet was looking up the local host's name. Due to a "bug" in the JVM,
this took 1+ seconds.
* Pre-flight test was too aggressive forcing GC on every servlet get/post.
Unfortunately in Java, if freeMemory is too small, one has no choice but to
force GC and check again.

I hope this is helpful.

Ken

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