Replication of pass-thru/RDM storage using Azure Site Recovery

Q. Can I replicate pass-thru/raw device mapped disks to Azure using Azure Site Recovery?

A. Pass-thru disks in Hyper-V enable a physical disk attached to the Hyper-V node to be directly mapped to a VM. This makes the disk no longer available to the Hyper-V host nor any other VM. Raw device mapping (RDM) is the VMware equivalent technology. Using pass-thru storage with Hyper-V is not recommended or even necessary with the new capabilities available with VHDX which supports up to 64 TB disks and can also be shared between VMs in Windows Server 2012 R2. Using a pass-thru disk breaks many features such as checkpoints (point-in-time captures of a VM), Hyper-V Replica and host-level backups.

Azure Site Recovery (ASR) uses a number of different channels to enable replication of workloads to Azure. One of those channels is Hyper-V Replica when the source is a VM running on Hyper-V 2012 or above. Using Hyper-V Replica it is not possible to replicate pass-thru attached storage. Another channel uses the Scout technology which was acquired through the InMage acquisition. Scout is an in-guest agent that captures writes to the disk and sends them ultimately to a target in Azure which writes the content to a VHD. When using the Scout channel, e.g. from VMware it is possible to replicate RDM disks for a VM (by treating the source as a physical source rather than VMware VM) and if in the future Scout also is used with Hyper-V this would enable pass-thru storage since Scout does not care about how disks are connected to a VM, it runs only in the actual guest OS. However a likely challenge is most pass-thru/RDM disks are very large and the maximum size for a replicated disk today is 1 TB, the maximum size for a disk in Azure which may mean even though technically an RDM from VMware could be replicated to Azure, the size of the disk may stop this working.

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