can i do mscs with an rdm that is in virtual compatibility mode across esxi hosts?
Sure I think that should work fine
Although possible, it's not recommended.
Cluster Virtual Machines Across Physical Hosts
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RDM in physical compatibility (pass-through) or virtual compatibility (non-pass-through) mode. VMware recommends physical compatibility mode. The cluster cannot use virtual disks for shared storage.
André
what would be the recommendation if I want to backup a mscs cluster with vdr? i cannot backup physical rdms.
I don't think snapshots on the cluster nodes will even work. Even if they would, consider the issues which could occur, if a snapshot remains active for whatever reason and the cluster fails over a disk to the other node. This other node - not aware of the snapshot - would use the base disk instead causing data corruption.
For such use cases you need to use guest OS agent based backups.
André