I've noticed that sometimes when a particular VM/ service is having issues, they all seem to hang. For example, I have a VM hosting my DNS (pihole) and another hosting my media server (jellyfin). If Jellyfin crashes for some reason, my internet in the entire house also goes down because it seems DNS is unable to be reached for a minute or so while the Jellyfin VM recovers.
Is this expected, and is there a way to prevent it?
Your services may run in separate VMs, but there are still some dependencies between them. You need to know, and think about, all the dependencies between your VMs.
For example, they share a common network interface (the one of the host machine). That is a dependency. If one VM is able to clog the network interface (and maybe your crashing one is doing exactly that), then it is clogged for all the other VMs too.
To resolve that dependency, you can either put another network interface card in your host machine and let only the pihole VM use it, or run the pihole on a real physical Pi.
You could also resolve the jellyfin's own problem. But resolving the dependency might give you a more reliable system.
That's a good point; My Virtualization server is running on a (fairly beefy) Intel NUC, and it has 2 eth ports on it. One is for management, and the other I plug my VLAN trunk into, which is where all the traffic is going through. I will limit the connection speed of the client that is pulling large video files in hopes the line does not saturate, and long term I'll try to get a different box where I can separate the VLAN's onto their own ports instead of gloming them all into one port.