clustering VirtualMachine or a Service ?

[Maaz]

we are running Oracle and Postfix cluster, and quite satisfied.

but people also run cluster of their Virtual Machines.(i.e instead of clustering service e.g Postfix, they cluster the vm on which postfix runs)

which approach is better ?
pros and cons ?

please share your knowledge and comments.

[Emil Velez · Accepted solution]

it works great especially if you do not have a application that works well with clustering software or you do not want to change how a app starts but just start it when system boots. This allows the application to be installed and started up the same way as a standalone system.

With Integrity virtual machine and ServiceGuard on HPUX you can move a clustered virtual machine from one VM host to another while the machine is running. The VM guests can be HPUX, VMS, Windows or Linux. I was impressed when I got it working plus there is a application monitoring api with the most recent release.

Not sure about clustering VMs on linux since I am a HPUX guy but it should be the same.

[Matti_Kurkela]

Emil summarized the pros of the VMs pretty well.

Cons:
If the physical hardware that is running the VM loses power or otherwise dies suddenly, the current run state of the VM will often be lost.

At this point, the VM must be rebooted on the failover host, which causes the usual filesystem checks, just like when crashing & rebooting a physical server. This can take a while.

If your SLA requirements are so strict that you would be considering Oracle RAC or similar active/active clustering solutions, a simple VM clustering solution may not be able to fail over fast enough in a hardware failure situation.

On the other hand, Postfix as such is a pretty lightweight service and email does not usually require guaranteed super-fast response times anyway. (Of course, external anti-spam and anti-virus functions can add significantly to the total workload).

If your Oracle database is a multi-terabyte, high-traffic monster, the performance penalty of virtualization might be important: you can get more raw performance from the hardware by leaving out the virtualization layer.

MK

[Maaz]

thanks for such a nice response and help... appreciated.


I would also highly appreciate, if you can provide me some document/url that compares(pros and cons) the two approach ?

Regards
Maaz