[CentOS] VM Philosophy 101 - How many and for what?

We are at the stage, finally, where we are prepared to
deploy public facing VM guests.  Now we have to answer
these questions: How many guests in total should we
contemplate and what services are placed on which guests?

My question comes down to whether it is considered
advisable to run the primary DNS and IMAP, or indeed all
services, on separate vm guests; or should we continue,
more or less, with the present split and just move
everything into guests on a one-for-one basis from our
existing hosts?  Will additional vms necessarily increase
the amount of time given to system maintenance as I
suspect?

Previously, with only a few physical hosts, the number of
platforms was fixed and services were split more or less
on the basis of internal and external users. Not to
mention which host had more available resources at the
time the service was implemented.

For example, presently our primary DNS and the IMAP
services runs on one server with MailScanner controlled
Sendmail used only for local delivery and forwarding.  On
another host we run the publicly accessible MX MTA and a
secondary public DNS.  On a third we run our fax server
and public web site together with a caching only DNS
service.

I very much would appreciate any relevant comments from
people who have already resolved this matter together with
their reasoning.

-- 
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James B. Byrne                mailto:ByrneJB at Harte-Lyne.ca
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