VMware HA Question

[Berg0]

What happens when VCenter is a VM and the host it is on loses network connectivity?

Does HA consider the other hosts to be in an isolation condition? would the other hosts then power off or shut down the guest VM's due to the isolation condition?

Would vCenter realize that the host it is on is the one in isolation, or would it think all other hosts dropped out of the cluster simultaneously, and begin attempting to bring up VM's with HA and likely encounter resource contention?

 

[SpaceHonkey]

The hosts only consider themselves in isolation when they can't ping their isolation address[es] (by default the vmkernel gateway).

Don't forget that vCenter does not actively direct hosts to bring up and down VMs in HA - it just keeps them up to date with the latest rules. They run autonomously of vCenter in this respect.

Does HA consider the other hosts to be in an isolation condition? would the other hosts then power off or shut down the guest VM's due to the isolation condition?

That depends on your cluster rules. What are your VMs set to do in isolation? Do they stay up? If so, the original host will still have a lock on the VM's files (can't remember the exact filename) which the next host will see when it tries to bring that VM up. It will instead back off and leave it alone (possibly for a time period before it tries it again).

Would vCenter realize that the host it is on is the one in isolation, or would it think all other hosts dropped out of the cluster simultaneously, and begin attempting to bring up VM's with HA and likely encounter resource contention?

Again, because vCenter is not 'directing' per se the HA operations, the scenario you outline won't happen. If you have the hosts set to shutdown in isolation, then after they shutdown (and close their file locks on the SAN), another host will eventually bring them up.

Now guys, correct me here if I'm wrong, but I'm pretty sure this is what'll happen.

Berg0, don't forget (as I'm sure you've already been lectured about before) but core services are not good candidates for virtualization for exactly the reasons you are concerned about here. DNS, DHCP, AD, and vCenter simply shouldn't be virtualized because the cluster relies on them to run.

 

[Thuleman]

Berg0, don't forget (as I'm sure you've already been lectured about before) but core services are not good candidates for virtualization for exactly the reasons you are concerned about here. DNS, DHCP, AD, and vCenter simply shouldn't be virtualized because the cluster relies on them to run.

I am pretty sure we had this discussion about this before and the conclusion was that especially core services should be virtualized since one can guarantee their uptime by means and HA and FT whereas you are SOL if your single physical machine experiences a problem.

 

[RiDDLeRThC]

I think as best pratices it would be good to have at least one instance of core services on a physical server and then if you wanted bring up a secondary on a virtual server.

In my office I have no choice but to do in VM because of budget but I know its limitations and what needs static IP's, host files, etc. It was fun having to enable EVC on my vCenter and yet having to shut it down to enable it. ended up having to move it into VMWare Server on a workstation and then migrate it back once I enabled what I needed in my cluster.

 

[Berg0]

a little more info might be required I guess

the scenario I'm looking at involves 3 VMware ESX hosts

They are primarily running web services, and some line of business software

Networking on all hosts is an aggregated pair of gigabit connections for VM traffic (8 different VLANs), and dedicated gigabit adapters on a separate VLAN for vMotion

Storage is 4GB fiberchannel HBA's on our SAN fabric (so VMs don't lose their vmdk's on the VMFS volumes if network goes out)

VMware HA host monitoring is enabled and set to power off.

I think I need to do a bit more reading on what constitutes as an isolation condition. I know VMware touts having Vcenter as a VM as a good thing, because it can be brought back up after host failure with HA

I think what might be throwing me for a loop is that my management interface is a VLAN on an aggregated link, and if the port channel goes down, the management interface is still up on the vswitch?

I think I've now confused myself. LOL

all of my test VMware hosts are running ESXi, I think i"ll need to fire up some 60-day demo ESX hosts and start trying to reproduce isolation conditions with HA enabled and see what happens when vcenter is on an isolated host w/o hardware failure etc.

 

[lopoetve]

The hosts only consider themselves in isolation when they can't ping their isolation address[es] (by default the vmkernel gateway).

Don't forget that vCenter does not actively direct hosts to bring up and down VMs in HA - it just keeps them up to date with the latest rules. They run autonomously of vCenter in this respect.



That depends on your cluster rules. What are your VMs set to do in isolation? Do they stay up? If so, the original host will still have a lock on the VM's files (can't remember the exact filename) which the next host will see when it tries to bring that VM up. It will instead back off and leave it alone (possibly for a time period before it tries it again).



Again, because vCenter is not 'directing' per se the HA operations, the scenario you outline won't happen. If you have the hosts set to shutdown in isolation, then after they shutdown (and close their file locks on the SAN), another host will eventually bring them up.

Now guys, correct me here if I'm wrong, but I'm pretty sure this is what'll happen.

Berg0, don't forget (as I'm sure you've already been lectured about before) but core services are not good candidates for virtualization for exactly the reasons you are concerned about here. DNS, DHCP, AD, and vCenter simply shouldn't be virtualized because the cluster relies on them to run.

This, although virting DNS/etc is fine, as long as you have a physical backup Just make sure everything isn't tied into the virtual infrastructure being up, because when it's not, you'll be hosed.