In general, when you have a VM on Azure, is it running on a single physical server or on many?
I see that there is an option to make sure that your VM is on a dedicated server.
What would happen if that server had a catastrophic failure?
Server Fault is a question and answer site for system and network administrators. It only takes a minute to sign up.Sign up to join this community
As virtual mentioned, regular VMs in azure are not tied to run on a unique physical host, they can be moved using what in hyper V is called "live migration" feature to any host in a pool of resourced defined by them (A very large pool from what i can tell). The storage is separated from the physical host so in a over-simplified way this mean that the virtual machines is no other thing that a large memory block, they freeze (quiesce) for a second that block, and then move it to another physical host. Unfreeze it and voila.
This live migration can be triggered by a number of reason, maintenance windows, lack of resources, or even certain host failures and You as a user, can trigger a Live migration anytime using the "redeploy option" in the azure console.
Regarding the second question,it's a new service in preview, there is no much info but based in experience with other providers, if you decide to provision only one host, if it fails, it fails. If you provision more than one host, the VM is moved to any of them.
The VM is generally run from a host machine. That host machine should be configured to use shared storage so Azure can live migrate your VM to a different baremetal host if they chose, which could happen once in a blue moon or very often without you knowing.
Not 100% with Azure specifically, but if the baremetal machine fails unexpectedly and they didn't have time to migrate your VM all you have to do is stop and restart your VM and it will be moved automatically to a new host.
Storage itself is usually separate as I mentioned earlier and it's usually at least 99.99% durable, if not 6-8 9's of durability.