I would like know the Cons if we built RAC on VM's, especially in terms of performance Query performance like physcial reads, writing, caching , RAC interconnecting comparing to physical system? We have plans to built the high transaction system (OLTP) i want to know the technical performance comparison between VM vs physical hosts in RAC?
while in generall this works (and I also did this a few times), the real question is why?
Oracle RAC provides everything, which a VM can provide (and even more) - no need to have live migration on VM level or anything else.
There are only 2 advantages especially Oracle VM will provide:
1.) Easier setup with the help of OVM templates.
2.) Hardware Partitioning.
So everything else a hypervisor will introduce is performance overhead. And especially if you do lots of I/Os and context switches (which is the case with an OLTP database), then the VM overhead will be recognizable.
My personal experiences with x86 hypervisors shows that it is likely around 5 to 20% compared to native hardware (even if you only run a single VM).
Don't ask what happens if you run multiple. Other (non x86) hypervisors and virtualisation (like Solaris Containers) may provide better numbers (close to physical hardware), but this also only counts for single VMs.
My advise: If you want performance, go physical....
And PS2: If you really think about OVM because easy setup and Hard Partitioning, I recommend to have a look at the Oracle Database Appliance (ODA). Which provides all that.... and good performance.
well SSD will increase performance - however the hypervisor uses CPU resources for the I/Os.
So SSDs on physical hardware will definitely increase the I/O better than on VMs....
So: Same arguments as above.