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Table 8 summarizes the various features and limitations of VMDKs and pRDMs, including where the multi-writer flag (MWF) is enabled for Oracle RAC.
Table 8. VMDK and pRDM features and limitations
VMware/ |
VMDK |
VMDK w/MWF |
pRDM |
pRDM w/MWF |
Notes |
Maximum ASM disk size |
62 TB |
64 TB |
|
||
Suspend VM |
Yes |
No |
Yes |
No |
Note 1 |
VM Snapshot – see independent persistent |
Yes |
No |
Yes |
No |
Note 2 |
VM vMotion |
Yes |
Yes |
Yes |
Yes |
|
VM High Availability (HA) |
Yes |
Yes |
Yes |
Yes |
|
VM Distributed Resource Scheduler (DRS) |
Yes |
Yes |
Yes |
Yes |
|
VMware Data Recovery (VDR) |
Yes |
No |
Yes |
No |
|
Storage vMotion |
Yes |
No |
Limited |
No |
Note 3 |
Virtual to physical migration |
Yes |
No |
Yes |
Yes |
Note 4 |
Guest OS SCSI target-based software |
Full |
No |
Yes |
Yes |
|
SAN features (snapshot, clone, replicate) |
Limited |
Limited |
Full |
Full |
Note 5 |
Hybrid RAC (VM and physical) |
N/A |
No |
N/A |
Yes |
|
Maximum RAC nodes |
N/A |
32 |
N/A |
100+ |
|
Note 1: Suspending VMs that are part of a cluster will result in node-eviction from the cluster.
Note 2: Making a snapshot of a VM with RDMs will result in the RDMs being converted to VMDKs.
Note 3: When migrating an RDM with Storage vMotion, the mapping file will be moved to the target datastore, but the volume will remain as before.
Note 4: Despite VMware documentation that states vRDMs cannot be presented to physical hosts, this is incorrect. Also note in testing, VMDKs have been successfully exported as iSCSI targets to physical hosts.
Note 5: You can utilize SAN based snapshots and clones of all volumes supporting VMFS datastores from outside of the guest OS without the use of tools such as AppSync, but such snaps and clones will be crash consistent, not application consistent.
VMFS and pRDMs offer advantages in a virtualized environment and careful thought should be given when selecting which technology to use.