Scale-out storage is a network-attached storage (NAS) architecture in which the total amount of disk space can be expanded as needed, even if some of the new drives reside in other storage arrays. If and when a given array reaches its storage limit, another array can be added to expand the system capacity. This approach may also be referred to as horizontal scalability.
Scale-out storage differs conceptually from the older scale-up approach, which may be also be referred to as vertical scalability. In a scale-up system, the network storage is confined to a single form factor, while in a scale-out system, new hardware is be added and configured as the need arises. The main advantages of a scale-out approach are cost containment more efficient use of hardware resources.
Before scale-out storage became popular, enterprises often purchased storage arrays much larger than needed in order to ensure that plenty of disk space would be available for future expansion. If that expansion never occurred or turned out less than expected, much of the originally purchased disk space went to waste. With a scale-out architecture, the initial investment can be more modest and if the storage requirement expands beyond expectations, new arrays can be added as needed, without limit.