Implementing VM High availability (HA) is a real challenge: first because you need to reliably detect when a server has really failed to avoid unpredictable behavior. But that's not the only one.
If you lose the network link but not the shared storage, how to ensure you will not write simultaneously on the storage and thus corrupt all your data?
We'll see how to protect your precious VM in multiple cases, and we'll illustrate that with real examples.
Even if you can have HA with only 2 hosts, it's strongly recommended to do it with at least 3 hosts, for obvious split-brains issues you might encounter.
HA requires far more maintenance and will create some traps if you are not aware. In short, it comes at a cost. Before using it, please carefully think about it: do you REALLY need it? We saw people having less uptime because of HA than without. Because you must understand what you are doing every time you are rebooting or updating a host.
The pool concept allows hosts to exchange their data and status:
- if you lose a host, it will be detected by the pool master.
- if you lose the master, another host will take over the master role automatically.
To be sure a host is really unreachable, HA in XCP-ng uses multiple heartbeat mechanisms. As you saw in the introduction, just checking the network isn't enough: what about the storage? That's why there is also a specific heartbeat for shared storage between hosts in a pool. In fact, each host regularly write some blocks in a dedicated VDI. That's the principle of the Dead man switch (opens new window). This concept is important, and it explains why you need to configure HA with a shared storage (iSCSI, Fiber Channel or NFS) to avoid simultaneous writing in a VM disk.
Here is the possibles cases and their answers:
- lost both network and storage heartbeat: host is considered unreachable, HA plan is started
- lost storage but not network: if the host can contact majority of pool members, it can stay alive. Indeed, in this scenario, there is no harm for the data (can't write to the VM disks). If the host is alone, i.e can't contact any other host or less than the majority, it decides to go for a reboot procedure.
- lost network but not storage (worst case!): the host considers itself as problematic, and start a reboot procedure (hard poweroff and restart). This fencing procedure guarantee the sanity of your data.
# Prepare the pool
You can check if your pool have HA enable or not. In Xen Orchestra, you'll have a small "cloud" icon in the Home/pool view for each pool with HA enabled.
You can enable it with this xe CLI command:
xe pool-ha-enable heartbeat-sr-uuids=<SR_UUID>
Remember that you need to use a shared SR to enable HA.
# Maximum host failure number
How many host failures you can tolerate before running out of options? For 2 hosts in a Pool, the answer is pretty simple: 1 is the maximum number. Why? Well, after loosing one host, it will be impossible to ensure a HA policy of the last one also fails.
This value can be computed by XCP-ng, and in our example case:
xe pool-ha-compute-max-host-failures-to-tolerate 1
But it could be also 0. Because even if you lose 1 host, is there not enough RAM to boot the HA VM on the last one? If not, you can't ensure their survival. If you want to set the number yourself, you can do it with this command:
xe pool-param-set ha-host-failures-to-tolerate=1 uuid=<Pool_UUID>
When you have more hosts failed equal to this number, a system alert is raised: you are in a over-commitment situation.
# Configure a VM for HA
This is pretty straightforward with Xen Orchestra. Go on your VM page, then edit the Advanced panel: you just have to tick the HA checkbox.
You can also do that configuration with xe CLI:
xe vm-param-set uuid=<VM_UUID> ha-restart-priority=restart
Before any update or host maintenance, planned reboot and so on, you need to ALWAYS put your host in maintenance mode. If you don't do it, XAPI will consider it as a unplanned failure, and will act accordingly.
If you have enough memory to put one host in maintenance (migrating all its VMs to other member of the pool), that will be alright. If you don't, you'll need to shutdown VMs manually from a XAPI client (Xen Orchestra or
xe), and NOT from inside the operating system.
You must be very careful before ANY maintenance task, otherwise HA will kick in and provide unpleasant surprises. You have been warned.
# Halting the VM
If you decide to shutdown the VM with
Xen Orchestra or
xe, the VM will be stopped normally, because XCP-ng knows that's what you want.
But if you halt it directly in the guest OS (via the console or in SSH), XCP-ng is NOT aware of what's going on. For the system, it seems the VM is down and that's an anomaly. So, the VM will be started automatically!. This behavior prevent an operator to shutdown the system and leave the VM unavailable for a long time.
# Host failure
We'll see 3 different scenarios on the host, with an example on 2 hosts, lab1 and lab2:
- physically "hard" shutdown the server
- physically remove the storage connection
- physically remove the network connection
lab1 is not the Pool Master, but the results would be the same (just longer to test because of time to the other host becoming the master itself).
Let's stay in our example of 2 hosts in a single pool. We configured the VM Minion 1 for HA, and this VM is running on the host lab1.
After each test, Minion 1 go back to lab1 to start in the exact same conditions.
# Pull the power plug
Now, we will decide to pull the plug for my host lab1: this is exactly where my VM currently runs. After some time (when XAPI detect and report the lost of the host, in general 2 minutes), we can see that lab1 is reported as Halted. In the same time, the VM Minion 1 is booted on the other host running, lab 2:
If you decide to re-plug the host lab1, the host will be back online, without any VM on it, which is normal.
# Pull the storage cable
Another scenario: this time, we will unplug the iSCSI/NFS link on lab1, despite Minion 1 is running on it.
So? Minion 1 lost access to its disks ad after some time, lab1 saw it can't access the heartbeat disk. Fencing protection is activated! The machine is rebooted, and after that, any
xe CLI command on this host will give you that message:
The host could not join the liveset because the HA daemon could not access the heartbeat disk.
Immediatly after fencing, Minion 1 will be booted on the other host.
lab1 is not physically halted, you can access it through SSH. But from the XAPI point of view, it's dead. Now, let's try to re-plug the ethernet cable... and just wait! Everything will be back to normal!
# Pull the network cable
Finally, the worst case: leaving the storage operational but "cut" the (management) network interface. Same procedure: unplug physically the cable, and wait... Because lab1 can't contact any other host of the pool (in this case, lab2), it decides to start the fencing procedure. The result is exaclty the same as the previous test. It's gone for the pool master, displayed as "Halted" until we re-plug the cable.