MSCS VMs and Snapshots

When using VMs in a MSCS cluster across box (CAB), you will need to setup the RDMs in a physical compatibility mode and enable bus sharing. Please note that VMs with RDMs in physical mode will not allow you to snapshot either. Basically, you will find that your MSCS VMs will have their snapshot option greyed out. What does this mean?

You can’t use VCB to backup your VMs as that relies on snapshots.
You can’t use vDR for backups as that relies on snapshots.
And lastly you can’t leverage the snapshotting ability for tasks like patching of your VM if you have been practicing that in the past.

When running the disk in independent/persistent mode, you would think that snapshots would still work and only snapshot the vmdk running the OS partition and not your RDM. However, with the bus sharing in place for MSCS to work, the snapshotting criterion is not met. Hence the option remains greyed out. Also, lets assume you are able to snapshot the VM somehow, and you take the snapshot of VM1, after keeping it for a day you suddenly decide to revert it back, I am not sure on how VM2 or even the cluster itself will behave when all of a sudden one of its nodes has forgotten what happened 24 hours ago. So, just don’t snapshot it even if you come up with a way of being able to do it.

One thing I haven’t tried yet is to see what happens when I turn a node off, snapshot it, do what I need to do and when I need to revert, turn it off and then revert. Not sure if this is even possible or supported. Not to mention, the bus sharing will be have to be disabled. But in all seriousness, I would never do this in a production environment. If I ever decide to test this, this will only be for my own curiosity.

Not being able to snapshot should not really be the reason for you to scrap MSCS in VMware. This is simply a limitation to understand prior to designing a solution. If snapshotting is of paramount importance, then this may not be your cup of tea, however will the alternate solution give you that ability? If not being able to back up the VM using VCB or vDR is your issue, then please be informed that you can still backup the VM by installing the agent inside the guest and use a traditional backup mechanism. Leverage the same infrastructure that has been backing up your physical world. MSCS in VMware is not perfect, but it will get there.

MSCS and vSphere Conflicts

As already addressed in the vSphere 4 u1 release notes, MSCS VMs are supported in a HA/DRS cluster, its amazing how many few have noticed the change. With all the functionalities that have been introduced over the years by VMware, its easy to miss a few things every now an then. Some consider MSCS a primitive form of clustering as opposed to HA/DRS clusters within ESX/i. However it must be noted that a HA/DRS cluster does not protect you from application failure or OS corruption. Neither does FT in vSphere. With a FT enabled VMs, it must be noted that when the primary VM blue screens, so does the secondary VM and you are left with two identical server both not functioning.

To sum it up, HA/DRS and even FT protects you from a hardware failure only. According to VMware, MSCS must be leveraged to maintain a 100% uptime for Windows guests. So what you can and cannot do with MSCS and VMware?

You can cluster two VMs on the same host, two VMs on seperates hosts and you can also cluster a physical and virtual machine. There are detailed guides published by VMware on how this can be achieved. (Click Here)

A 50K foot view of what you can and cannot do and this will also differ based on the version of ESX/I you are running:
Only two nodes in a MSCS cluster
MSCS cannot be an FT enabled VM
Though MSCS VMs can be in a HA/DRS cluster, both HA and DRS should be disabled for all the VMs that are a part of MSCS
Quorum and shared disk should not have the VMFS signature and should be presented to all the hosts in the cluster where the MSCS VMs reside (Think about it, it makes sense)
Don’t overcommit and try to create a reservation for your VM equal to the size of the memory assigned.
The VMware doc will have more details

Now the last part, DRS is disabled because under the hood, HA uses vMotion. Though vMotion is rapid and causes no outage for the users, MSCS heartbeat is very sensitive and may detect the few seconds of the stunning period as a node failure and consider that node to be down. This is certainly not what you want. Hence its best not to vMotion, which is why DRS is disabled as well.
Why is HA disabled? No one has been able to give a straight answer on that and it basically comes down to that its not supported.

As of now I really don’t know why you can’t have HA enabled for a VM that is part of a MSCS cluster.
The good news is, with 4 u1 and onwards, you can utilize the same hosts that are in a HA/DRS cluster to run your MSCS VMs, just don’t forget to disable these features for the VMs that are part of the MSCS cluster or else the VMware and MS support may stiff you in time of need.


VMware vSphere 4.1 HA and DRS technical deepdive — eBook

After waiting forever, Duncan and Denneman have finally released the eBook version of their famous “VMware vSphere 4.1 HA and DRS technical deepdive” book. I bought a hard copy last year but never got around to fully dedicating my time to reading it, mainly due to laziness. Now with it being an ebook, you have several new options to enjoy. As of now its being released on kindle only. iPad owners can install the kindle app and still enjoy this book on their superior tablets.

I love how you can simply click on the hyperlinks provided in the book now. Thanks Duncan Epping and Frank Denneman!. Without further due, please purchase your copy at VMware vSphere 4.1 HA and DRS technical deepdive for the dirt cheap price of $7.50. This will be the best investment you make in gaining more VMware knowledge. Enjoy!