Citrix XenServer pools allow you to view multiple servers and their connected shared storage as a single unified resource, enabling flexible deployment of virtual machines based on their resource needs and business priorities. A pool may contain up to 16 servers running the same version of XenServer software, at the same patch level, and with broadly compatible hardware - see Pool Requirements for details of hardware and configuration prerequisites.
One server in the pool is designated as the pool master, and provides a single point of contact for all of the servers in the pool, routing communication to other members of the pool as necessary.
If the pool master is shut down, the pool will be unavailable until the master is rebooted and back on line or until you nominate one of the other members as the new pool master. Every member of a resource pool contains all the information necessary to take over the role of master if required. On an HA-enabled pool, a new pool master is automatically nominated if the master is shut down.