ZFS is not just a file system -- it is also a volume manager. Storage devices are organized by ZFS into pools, which are collection of hardware devices, either partitions or whole disks. Pools are then further divided into virtual devices, or vdevs. A vdev can be either a stripe, mirror (with multiple redundancy levels) or a raidz(with multiple redundancy levels) device. If a pool has multiple vdevs, then data are striped across all vdevs equally.
All servers at Durham Tech use a single storage pool. The main Colleague server uses four drives split into two mirrored vdevs, while the backup server uses two drives in a single mirrored vdev.