There isn’t a single rule for this – there are more like thousands of rules! Basically have an idea of what workloads VMs are generating in terms of IO and try to balance them out but also bear in mind that write intensive loads will perform better on RAID 10 than on RAID 5 but RAID 10 uses more disks than RAID 5 does.
Whilst not specifically related to RAID and it talks about EMC storage, Optimal VM Placement offers some interesting thoughts and mentions the alarms that can be set in vCenter that are usueful to monitor problems:
- VM Disk Usage (KBps)
- Total Disk Latency (ms)
- VM Disk Abort
- VM Disk resets
Also, as a rule of thumb, if a server consistently generates a certain number of IOPS as either reads or writes on physical hardware, it will probably generate the the same on virtual hardware. So it follows that if you’d use RAID 10 for that physical server, you should use a RAID 10 LUN with the virtual server. It’s a common sense thing gained from experience really.