Ok, here's the clarification:
A machine can be a database server or a fileserver, or both.
You have to have at least one machine providing database service. It is
preferrable that you have multiple machines -- either 3 or 5 -- 3 is
usually sufficient. It's important that they be an odd number of
machines, as in the case of a server failure you need a quorum of
servers still talking to each other to sort out who's database is writable.
You may choose, as I said, to run the database service on machines that
are also fileservers. I would recommend, however, that you run separate
database server machines. We currently use three sun Netra X1 systems.
Not too expensive, not very powerful, but they perform the job quite
well -- and we have a pretty decent sized cell.
There are many reasons it's a good idea(tm) to run the database services
on their own machines. First, the IP addresses of these machines should
never have to change during the lifetime or your cell (well, they can,
but then you have to update your cellservDB information, etc. A pain in
the butt.) We've had generations of fileserver machines come and go,
but the database servers stick around. Second, a functional database
service is super super important to the operation of any AFS services.
You can have fileservers down, and still access files in AFS that were
not served from those servers -- but if your DB services go away...
DOOM. It's just good practice to isolate services like that.
Hope that helps.
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