You enable HA for a resource pool using the Configure HA wizard. The wizard takes you through the HA configuration process, step-by-step, calculating the server failure limit for the pool given the available resources and the HA restart priorities you specify.

To open the Configure HA wizard: in XenCenter, select the pool, click on the HA tab, and then click Configure HA.

Alternatively:

• On the Pool menu, click High Availability.
• Right-click in the Resources pane and then click High Availability on the shortcut menu.

To configure HA on a pool:

1. Ensure that the HA requirements identified in HA requirements are satisfied.

2. Open the Configure HA wizard and click Next on the first page of wizard to scan the pool for a shared iSCSI, Fibre Channel or NFS LUN that can be used as the pool’s heartbeat SR. If no suitable SR is found, you will need to configure some appropriate new storage before you can continue.

3. On the Heartbeat SR page, choose an SR from the list and then click Next.

4. On the HA Plan page, select one or more VMs in the list and set the required VM startup settings. For more on these options, see VM startup settings. To select more than one virtual machine, click at the start of the selection, scroll to the end of the selection, and then hold down SHIFT while you click where you want the selection to end. To select several virtual machines that are not next to each other in the list, click the first one, press CTRL, and then click the additional virtual machines that you want to select.

   Option

   Description

   HA restart priority

   Choose a restart priority for each VM:

   • Choose Restart to ensure the selected VM(s) are restarted if sufficient resources are available within the pool.
   • Choose Restart if Possible if it is not essential to restart the VM automatically.
   • Choose Do Not Restart if you never want the VM to be restarted automatically.

   Restart order

   Specifies the order in which individual VMs will be started up during the HA recovery operation, allowing certain VMs to be started before others. VMs with a restart order value of 0 (zero) will be started first, then VMs with a restart order value of 1, followed by VMs with a restart order value of 2, and so on.

   Attempt to start next VM after

   This is a delay interval that specifies how long to wait after starting the VM before attempting to start the next group of VMs in the startup sequence, that is, VMs with a lower start order.

5. Also on the HA Plan page, under Server failure limit, you can set the number of server failures to allow within this HA plan. This value should be less than or equal to the maximum failure capacity for the pool, shown here as max. If max is 0 (zero), the pool is overcommitted, and you will not be able to continue to the next page of the wizard without either adjusting the HA restart priorities or making more resources available within the pool; see To increase the maximum failure capacity for a pool. Click Next when you have finished HA plan configuration.

6. On the last page of the wizard, review your HA configuration settings. Click Back to go back and change any of the settings or click Finish to enable HA and close the wizard.

Good pdf about citrix HA is found here.