[CentOS] How to organize your VMs

Sat Apr 10 16:59:14 UTC 2021
Simon Matter <simon.matter at invoca.ch>

> Hi,
> Up until now my main production server has been a "bare metal"
> installation of
> CentOS 7.9 hosting a variety of stuff.
> * DNS server with BIND for eight domains
> * IMAP mail server with Postfix and Dovecot for these domains, with about
> two
> dozen mail accounts
> * Webmail with Roundcube for all the mail accounts
> * Various WordPress-based websites and blogs
> * Several instances of the management software Dolibarr
> * The learning platform GEPI for our local school
> * One instance of OwnCloud for half a dozen users
> The hardware has no problems to deal with all that performance-wise. But
> managing all this in one big bulk has become a bit of a problem, since the
> LAMP-based PHP applications (WordPress, Dolibarr, GEPI, OwnCloud)
> increasingly
> cultivate their idiosyncrasies, so this feels more and more like herding
> cats.
> My main goal in migrating all this stuff preogressively to a series of
> neat VMs
> hosted on a KVM hypervisor is clarity and ease of maintenance.
> Now I wonder what could be a smart subdivision of all these VMs. After a
> bit of
> brainstorming, here's what I can come up with.
> 1. It would make sense to regroup all the applications, e. g. one VM for
> all
> the Dolibarr hostings, and then a different VM for WordPress, and a third
> VM
> for OwnCloud.
> 2. It's tempting to have a lot of small VMs for clarity's sake. On the
> other
> hand, it's maybe better to have one single VM for all the mail stuff.
> 3. Should I put all the Roundcube instances in a separate VM? Or does that
> go
> with the Postfix/Dovecot mail VM?

I'd suggest to have it on one VM. I guess Webmail and the other parts
don't disturb each other and they really belong together, so why not put
them into one instance.

> 4. DNS is a bit of a special case, a bit of a catch 22. I would be tempted
> to
> setup an extra (bare-metal) machine for just handling this. Since BIND
> provides
> the DNS information about the hypervisor and the backup server themselves
> this
> becomes a bit of a chicken-and-egg situation.

If the backup server and the KVM host are two hardware servers, then why
not put one DNS server on each of them? Primary on one and secondary on
the other hardware so as long as one of these hosts are up, you have
working DNS.

> 5. Even if it's tempting to multiply VMs, let's not forget that I have to
> keep
> an eye on hardware resources, not to forget I have to pay for every extra
> IPv4
> address.

Why not have some hosts with only internal addresses? I don't think all of
the hosts will need public addresses, right?


> I'd be curious to have your input, since I'm fairly new to this sort of
> approach.
> Cheers,
> Niki
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