Not Virtualizing Everything, or Know Your Dependencies

I’ve commented before on how I generally object to plans to virtualize absolutely everything in a data center. It looks like Eric Siebert agrees with me in his article on recovering servers after power failures:

Virtual servers can be even more problematic. If you have all your DNS servers virtualized which cannot be started because of network or shared storage issues, you can run into problems starting other servers and services that rely on DNS. Consider having at least one physical DNS server or having one or two DNS servers running on local storage instead of shared storage.

Running services from VMs might be a great idea, especially if you’re using techniques like anycast DNS. It’s also pretty useful for creating test environments for your infrastructure services. The thing is, VMware uses DNS, so if you don’t have a physical box running DNS and DHCP you have a dependency loop. Likewise if your SAN gear needs DNS to start, or you connect to your iSCSI volumes using DNS names. Having at least one physical machine running DNS and DHCP makes life a lot easier in a crisis.

Moral of the story is twofold: you need to know what your dependencies are and account for them, and VMware has dependencies like everything else.

1 thought on “Not Virtualizing Everything, or Know Your Dependencies”

  1. That’s a very good point that I hadn’t considered before now. Good call.

    I’ve got my machines set up so that the primary (or top listed, anyway) dns server is the local one, and the 2nd is in a remote location, so theoretically this wouldn’t be an issue.

    Of course, we covered theory a couple of stories down

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