Mark Achtemichuk over at VMware published a great article the other day on the use of VMDKs (Virtual Machine Disk) versus RDMs (Raw Device Mapping) in VMware vSphere 5.1. If you’re at all interested in virtualization storage performance in VMware I’d encourage you to read it. The bottom line is that Mark (and VMware as a corporate entity given past literature on this subject) would rather people use VMDKs to encapsulate the VM rather than using RDMs.
Because Unitrends offers a VMware-based virtual appliance for backup, archiving, replication (disaster recovery), and failover virtualization (instant recovery), we see the issue from both the application side as well as the protection side. We also quite often run into virtualization administrators who passionately believe in the use of VMDKs and others who just as passionately believe in the use of RDMs.
From a backup perspective, RDMs pose an interesting challenge for many backup vendors. There are fundamentally two types: physical (also called pass-through) and virtual (also called logical.) Most HOS-only backup vendors (vendors who call VMware’s VADP (vStorage APIs for Data Protection) aren’t able to protect physical RDMs. The reason is that physical RDMs can’t have VMware snapshots taken against them (the same limitation as VMs that have independent disks.)
One reason that I’m such a fan of heterogeneous backup (backup that operates as needed at both the HOS- and GOS-level) is that it offers choice. There are reasons to use physical RDMs that involve clustering (in particular, MSCS clustering that spans physical hosts), direct storage commands to SAN management software, and the like. Constraining choice definitely makes things simpler – but all too often those constraints can significantly hinder the best results for IT administrators.
What do you think – have you run into situations in which RDMs made your life a lot easier or showed significantly better performance or do you think VMDKs are always the best answer?