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Subject: Re: How large on one server?
From: Brad Knowles (blkskynet.be)
Date: Tue Jun 13 2000 - 10:46:24 CDT

At 4:26 PM +0200 2000/6/13, Gustav Foseid wrote:

> Does anyone have any experience on when it is necessary to migrate from a
> single server to a cluster of mailservers?

        It depends.

> This will of course depend on
> what kind of users you have, but I have found very little information on
> how large a system has to be before a migration to a cluster is needed.

        The answer is: it depends.

> I have two systems where I have to make this decission:
>
> - One ISP system with approx. 100.000 -- 150.000 users.
>
> - One system with approx. 20.000 business users (typical mail load is busy
> hour of 8.000 mails and a notable percentage connecting to POP every 5
> minutes or using IMAP).
>
> Assume Intel servers, Sun E250 or similar...

        You can ask for anecdotal evidence all you want, but the problem
is that there are so many different factors that could possibly be
involved, that it is simply impossible to give you any figures
without having so many caveats around the statements as to make them
meaningless.

        For example, we serve over 600,000 "Free Internet" users on a Sun
E420R with four 450Mhz processors, 4GB of RAM, Solaris 7, Veritas
VxFS, an external RAID array (Hitachi/Comparex D1400), and apache
with a custom webmail interface. We have another identical machine
to handle the hundreds of thousands of paying customers we have
(analog modem, ISDN, ADSL, dial-ups, as well as leased-line).

        But even this level of detail is pretty much useless. You don't
know what our mix of "heavy" users is compared to our "light" users,
you don't know how many users we have logged on at any one time and
how this compares to your utilization, you don't know how our
customized versions of a POP3 and/or IMAP server and a local delivery
agent would compare against what you might be running, and you don't
know what kind of an impact our webmail front-end places on the
system (and how that would compare to a system that didn't have to
run a webmail interface).

        Simply put, there are just too many variables here to contend with.

        If you'd like to talk to some people who specialize in doing
consulting in this area, send me your details via private e-mail, and
I'll be glad to put them in touch with you. 

--
   These are my opinions -- not to be taken as official Skynet policy
======================================================================
Brad Knowles, <blkskynet.be>                || Belgacom Skynet SA/NV
Systems Architect, Mail/News/FTP/Proxy Admin || Rue Colonel Bourg, 124
Phone/Fax: +32-2-706.13.11/12.49             || B-1140 Brussels
http://www.skynet.be                         || Belgium