Replica or iscsi?

Question

System administration

Good morning! I can not decide whether a replica is needed in my case? There are two hyper-v hosts, one is load, the second is spare. Ordinary computer. Planned to do replication, the controller of the main and the second from the main host to the spare machine.
But now I'm asking myself, is it necessary? On the main host, the virtual machines are on the iscsi network drive of the Synology NAS. The backup controller will not save, there will be a loss of trust. And replication and iscsi are not interchangeable technologies?

Answer 1

There are two different tasks - data protection and process continuity protection, i.e. to keep your application running.
Each backup tool protects against its own threat model, most often these are hardware problems and connection problems, but there is, for example, a human factor, the administrator broke something, deleted it, configured it incorrectly, etc. This also needs to be protected somehow.
This is easier to achieve by means of the application itself, when it itself is ready for the fact that it can be restarted, moved between hosts, data is backed up, logic is duplicated (when the same task is sent to several hosts at once, and then the response is compared), logs are collected that can be ' lose' and restore the situation to the right moment in time, etc.
There are OS-level solutions to ensure business continuity against hardware failures through virtual machines and distributed NAS.
Depending on how much you are willing to spend on ready-made solutions or implementing your own, choose a cluster file system (from free DRBD, as you understand Linux) or set up mirroring (software raid over iscsi for example) and manually restart the central node (requires a restart and machines launched from them, with reservations). In any case, the NAS node should not be combined with the software node on which the applications are actually launched (with reservations, the disks themselves can be there, distributed by software iscsi or nbd or aoe ...), the divide and conquer rule applies here.
Virtual machines have High Availability (kvm / xen / vmware / ...), which allows you to quickly move a virtual machine within the same cluster, when both machines in it can be geographically located in different places (the machine is suspended, the memory is copied, restored ).
... and even do it on the fly, synchronizing the RAM between the master and slave copies of the machines (then moving between nodes in the cluster will take seconds) - this by itself imposes high requirements on the network. These are expensive features of adult virtual machines, vmware
ps, unfortunately, choose two out of three - fast, reliable, cheap

Answer 2

No, absolutely not interchangeable. Only replication is suitable for a domain controller.

Answer 3

Transfer the second controller (domain) to the second host as a VM, replication will remain, and if anything, two hosts will not go down at once.