That’s a mouth full of abbreviations for a title, isn’t it?
What the vMSC allows us to do, is to create a regular stretched vSphere cluster, but now also stretch out the storage between the two clusters. This can be done in two ways (to quote from Duncan’s article):
I want to briefly explain the concept of a metro / stretched cluster, which can be carved up in to two different type of solutions. The first solution is where a synchronous copy of your datastore is available on the other site, this mirror copy will be read-only. In other words there is a read-write copy in Datacenter-A and a read-only copy in Datacenter-B. This means that your VMs in Datacenter-B located on this datastore will do I/O on Datacenter-A since the read-write copy of the datastore is in Datacenter-A. The second solution is which EMC calls “write anywhere”. In this case VMs always write locally. The key point here is that each of the LUNs / datastores has a “preferred site” defined, this is also sometimes referred to as “site bias”. In other words, if anything happens to the link in between then the storage system on the preferred site for a given datastore will be the only one left who can read-write access it.
The last scenario described here is something that obviously can cause some issues. EMC tried to address this by introducing the “independent 3rd party”, in form of the VPLEX Witness. Some documentation states that this witness should run in a 3rd site, but I would recommend to run this in a separate failure domain.
In essence, we have created the following setup:
Awesome stuff, because we can do new things that weren’t quite possible before. Since VPLEX is one of the key storage virtualization solutions from EMC that allow us to perform an active/active disk access, we can perform a vMotion between the two sites, and due to the nature of VPLEX, we also perform a sort of storage vMotion on the underlying disks. That, without you having to shut down the VM to do both things at the same time. Pretty neat!
Now, as Chad describes here, a new disk connectivity state was introduced with vSphere 5, called “Permanent Device Loss” or PDL. This was a great feature to communicate to your infrastructure that a target was intentionally removed. You could unmount the disk, and remove the paths to your target in a proper way.
It was also useful to indicate an unexpected loss of your target, indicating that your cluster is in a partitioned state. The problem here was that a PDL state and VMware HA didn’t work so well together. When you had an APD notification, HA didn’t “kill” your VM, and your virtual machine would usually continue to respond to pings, but that was about it.
Then along came vSphere 5 Update 1, which allows us to set a flag on each of the hosts inside our cluster, and set a different flag for our HA cluster. Now, we can actually use HA and see terminate the VMs and have it restart the virtual machines on the hosts in our cluster that still have access to their datastores in their respective preferred sites.
I’ve created a short (ok, 8 minutes) video that show exactly this scenario. You’ll get a quick view of the VPLEX setup. You’ll see the Brocade switches that will change from a config with the normal full zoneset, being switched to a zoneset that will disable the inter-switch links between both VPLEX clusters. And you’ll see the settings inside of my vSphere lab setup, with the behavior of the hosts and virtual machines.
Since I’m quite new to creating videos like this, I hope the output is acceptable, and the video is clear enough. If you have any questions, feedback or would like to see more, please leave me a comment and I’ll see what I can do.
Just a quick modification to my post, since it wasn’t actually VM-HA (or VM monitoring) responding to the PDL event, but HA terminating the VM when running in to the PDL state, as Duncan pointed out to me on Twitter. Sorry for any confusion I may have caused!