Another way of enabling efficient backups is to pool the tape devices: In a large enterprise SAN there can be a large number of data volumes which need backing up to a much smaller number of tape devices. This can make backup administration complex. The solution is to have a mechanism for pooling available tape resources, so that the backup application requests a logical tape, and the virtual tape manager maps that onto a physical tape. As with disk storage virtualization, tape virtualization allows multiple hosts to access storage resources without regard to the location or properties of the physical storage devices. In addition, some such virtual tape managers can have a large cache of disk storage—typically several tapes’ worth—in order to manage occasions when more tapes are requested than there are tape devices available, and in order to minimize the effective backup time. For such tape pooling to work robustly, it must be integrated with the backup software.