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Notes on vSphere High Availability (HA)

Just some notes on vSphere HA as I reading along on that. Nothing new here …

Starting with vSphere 5.0 HA has a Master/ Slave model. One ESXi host is elected as a Master, the rest are Slaves. The Master is the one with the most number of datastores connected to it; if all ESXi hosts have the same number of datastores connected to it, the Master is the one with the largest Managed Object ID (MOID). Note that the MOID is interpreted lexically – so an MOID 99 is larger than 100. PowerCLI can be used to view the MOIDs:

Also, the MOID is a vCenter specific construct. Whenever a host, VM, datastore, etc is added to vCenter it is assigned an MOID. For instance here are the MOIDs of my datastores:

Although I haven’t used this it’s also possible to find MOIDs vSphere Managed Object Browser. See this KB article for more info.

Back to the topic – the above is how a Master is elected. There’s only one Master per cluster. When it comes to HA, the Fault Domain Manager (FDM) on this Master is responsible for most of the tasks (which is why even if vCenter is down for a while HA can continue working). vCenter checks with the Master and the Master communicates with vCenter to keep each other abreast of the cluster situation.

  • FDM is installed at /opt/vmware/fdm/fdm/
  • FDM config files are at /etc/opt/vmware/fdm/

The Master monitors the Slave hosts and if a Slave goes down/ is unreachable the Master is responsible for starting these Protected VMs elsewhere. The Master is also responsible for keeping the Slaves abreast of the cluster configuration.

Slaves are limited to monitoring VMs running with them. Slaves monitor the VM health and if a Protected VM powers down they inform the Master so it can be restarted. (Note on Protected VMs: once you enable VM monitoring on a cluster or set a VM as Protected, the VM must be powered off and powered on to be protected). Slaves also keep in touch with each other and if they find the Master is down they conduct an election to select a new Master.

The only time vCenter communicates with Slaves is when a new Master needs to be elected or when the Master reports a Slave as missing and so vCenter tries to contact it.

Slaves send network heartbeats to the Master every second. When a Master stops receiving heartbeats from a Slave it knows it is offline or partitioned/ isolated. Similarly when a Slave stops receiving heartbeats from a Master it knows the Master is offline or partitioned/ isolated.

  • If a Slave is cut off from all other hosts (Master and Slaves) it is considered isolated (caveat: you can also specify up to 10 isolation IP addresses to ping – if these are reachable but the Master and Slaves are not, the Slave does not consider itself isolated, only partitioned).
  • If a Slave is cut off from the Master and some/ none Slaves (i.e. it still has contact with some Slaves) then it is considered partitioned.

In the past if a Slave were isolated/ partitioned the Master would consider it as offline and restart its Protected VMs elsewhere. Starting with vSphere 5.0 the Master also sends a ping (ICMP packet) to the Slave to see if responds and uses datastore heartbeats to verify the Slave is really down. It could be that the Management network is down but the VM and storage networks are up, so the VMs are still functioning as expected.

Datastore heartbeats work thus (and remember they are only used in case of isolation/ partition scenarios):

  • When enabling HA for a cluster, a datastore is automatically selected (or can be selected manually by the user) to be used for datastore heartbeats.
  • On this datastore a folder called .vSphere-HA is created within which a sub-folder of name FDM-<Fault Domain ID>-<vCenter Server Name> is created. (Such a name allows the same datastore to be used by multiple clusters).
  • Each host creates a file with its MOID name in this sub-folder. Like thus:heartbeats
  • Notice the host-X-hb file above? That is created by each host (you can check the /var/log/fdm.log file on each host to see it creating this file). When a Slave does not get heartbeats from a Master it updates its file above (and also checks the timestamp of the file for the Master – if that has updates it means the Master is alive). Similarly, when a Master does not hear from a Slave it checks the Slave’s file above to see if there’s updates. This is how datastore heartbeats work.
  • If a Slave is network partitioned – i.e. it cannot contact the Master – but can see some of the other Slaves, the Master and Slave can conclude that each other is still alive from the datastore heartbeats as above.
    • If the Master is down – i.e. the Slaves think they are partitioned because actually the Master is down – they can now elect a new Master since there are no datastore heartbeats from the Master.
    • If the Slave is down – i.e. the Master is not getting any datastore heartbeats from the Slave – then it restarts the Protected VMs on other hosts. (If the Slave were actually up but had lost network access to the datastore and so cannot update heartbeats, it is as good as down because the VMs have probably crashed by now).
  • If a Slave is network isolated – i.e. it cannot contact the Master or any other Slave (nor can it ping the isolation addresses) – then the Slave adds a special bit in the host-X-poweron file above. This tells the Master that the Slave is network isolated.
    • The Master then locks the file called protectedlist. This is a list of all Protected VMs. Once the Master has locked this file, the Slave knows the Master has taken responsibility for the Protected VMs and the Slave can leave these powered on, shut down, or power off (depending on which of these is selected as the host isolation response when setting up HA).
    • The protectedlist file thus ensures that unless another host has taken over these VMs the current host will not shut down/ power off these.

Two advanced options to keep in mind:

  • I mentioned this earlier: das.isolationAddress[0-9] allow one to specify up to 10 isolation IP addresses to check before a host considers itself isolated.
  • And das.allowNetwork[0-9] allow one to specify up to 10 port groups to use for HA. See this KB article for examples.

Lastly, I haven’t read it fully but this HA Deepdive is a great resource.

Notes on vSphere High Availability (HA) by rakhesh is licensed under a Creative Commons Attribution 4.0 International License.