Chris Hahn and I recently put together what we thought were the pros and cons for attaching a LUN to ESX and then using RDMs or just attaching an iSCSI LUN directly to the Guest VM via the MS initiator. In our example, we were connecting back to EMC storage so EMC PowerPath also became part of the equation. I thought I would summarize the discussions and jot it all down in a blog post to solicit some additional opinions on the subject.
- No additional PowerPath software cost
- No guest software to install.
- No extra storage management on the guest level
- Most widely deployed solution for connecting ESX guests directly to LUN.
- Easy migration to VI4 which will most likely support PowerPath, and multiple connections per iSCSI target
- RDM LUNs must be presented to all ESX hosts in cluster to facilitate vMotion, HA and DRS.
- No load balancing for individual LUNs (ESX will establish a connection per iSCSI target, so total storage traffic can be balanced, but traffic for a single LUN cannot be balanced across multiple NICs)
PowerPath with Microsoft iSCSI initiator:
- iSCSI guest traffic could be moved to a dedicated vSwitch, allowing for load balancing and best possible performance
- PowerPath would facilitate multipathing within the iSCSI and allow for better performance.
- Application support may be more familiar with windows guest storage connectivity support, with this configuration it would be managed the same way as physical windows hosts.
- Dedicated NICs for PowerPath would take away from current ESX NICS (either NICS for VM traffic, or ESX iSCSI). It could also share NICs with the ESX iSCSI vSwitch, but that could impact the performance benefits.
- Storage must be individually configured within each guest
- SRM is not supported
Ultimately, the decision seems to weigh between Site Recovery Manager support and performance benefits. Long term, with PowerPath likely baked into vSphere and performance no longer an issue, it would seem that RDMs would be the preferred choice.