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best-effort priority setting are not part of the failover plan and are not guaranteed to be kept running, since
capacity is not reserved for them. If the pool experiences server failures and enters a state where the number of
tolerable failures drops to zero, the protected VMs will no longer be guaranteed to be restarted. If this condition
is reached, a system alert will be generated. In this case, should an additional failure occur, all VMs that have a
restart priority set will behave according to the best-effort behavior.
If a protected VM cannot be restarted at the time of a server failure (for example, if the pool was overcommitted
when the failure occurred), further attempts to start this VM will be made as the state of the pool changes. This
means that if extra capacity becomes available in a pool (if you shut down a non-essential VM, or add an additional
server, for example), a fresh attempt to restart the protected VMs will be made, which may now succeed.
Note:
No running VM will ever be stopped or migrated in order to free resources for a VM with
always-run=true to be restarted.
Enabling HA on a XenServer Pool
HA can be enabled on a pool using either XenCenter or the command-line interface. In either case, you will
specify a set of priorities that determine which VMs should be given highest restart priority when a pool is
overcommitted.
Warning:
When HA is enabled, some operations that would compromise the plan for restarting VMs
may be disabled, such as removing a server from a pool. To perform these operations, HA can
be temporarily disabled, or alternately, VMs protected by HA made unprotected.
Enabling HA Using the CLI
1. Verify that you have a compatible Storage Repository (SR) attached to your pool. iSCSI, NFS or Fibre Channel
are compatible SR types. Please refer to the reference guide for details on how to configure such a storage
repository using the CLI.
2. For each VM you wish to protect, set a restart priority. You can do this as follows:
xe vm-param-set uuid=<vm_uuid> ha-restart-priority=<1> ha-always-run=true
3. Enable HA on the pool:
xe pool-ha-enable heartbeat-sr-uuids=<sr_uuid>
4. Run the pool-ha-compute-max-host-failures-to-tolerate command. This command returns the maximum
number of hosts that can fail before there are insufficient resources to run all the protected VMs in the pool.
xe pool-ha-compute-max-host-failures-to-tolerate
The number of failures to tolerate determines when an alert is sent: the system will recompute a failover
plan as the state of the pool changes and with this computation the system identifies the capacity of the pool
and how many more failures are possible without loss of the liveness guarantee for protected VMs. A system
alert is generated when this computed value falls below the specified value for ha-host-failures-
to-tolerate.
5. Specify the number of failures to tolerate parameter. This should be less than or equal to the computed
value:
xe pool-param-set ha-host-failures-to-tolerate=<2> uuid=<pool-uuid>
Removing HA Protection from a VM using the CLI
To disable HA features for a VM, use the xe vm-param-set command to set the ha-always-run parameter
to false. This does not clear the VM restart priority settings. You can enable HA for a VM again by setting the
ha-always-run parameter to true.