Parent was probably talking about the application server, which is the typical example of K8s usage, whereas most databases don't fit well there and are usually separate from the K8s cluster.
Yeah. They must be. I’ve had several k8s enthusiast dissuade from putting a db in k8s for performance reasons. This may change overtime but I believe there is a performance bottleneck in disk I/o (or at least that’s how it was explained to me)
That really depends on your k8s environment. If you're running on bare metal and use host bind mounts for storage, you won't have a problem. But as soon as you introduce shared storage it gets messy...