The grid storage facade

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Lately the term "grid storage" has crept into the product literature of vendors ranging from storage stalwarts IBM and Network Appliance to numerous start-ups. While grid storage appears to borrow conceptually from grid computing - a set of technologies used to build supercomputers from clusters of inexpensive processors - the similarity ends there. The two have little else to do with each other.

Grid storage refers to two items: a topology for scaling the capacity of network-attached storage (NAS) in response to application requirements, and a technology for enabling and managing a single file system so that it can span an increasing volume of storage.

One way to view grid storage is as a means to scale NAS storage horizontally and vertically while avoiding the problems associated with each.

Currently, scaling horizontally means adding more NAS arrays to a LAN. This works until the number of NAS boxes becomes unmanageable. In a "grid" topology, NAS heads are joined together using clustering technology to create one virtual head. NAS heads are the components containing a thin operating system optimized for Network File System (NFS) protocol support and storage device attachment.

Conversely, the vertical scaling of NAS is accomplished by adding more disk drives to an array. Scalability is affected by NAS file system addressing limits (how many file names you can read and write) and by such physical features as the bandwidth of the interconnect between the NAS head and the back-end disk. In general, the more disk placed behind a NAS head, the greater the likelihood the system will become inefficient because of concentrated load or interconnect saturation.

Grid storage, in theory, attacks these limits by joining NAS heads into highly scalable clusters and by alleviating the constraints of file system address space through the use of an extensible file system.

Who needs grid storage?

Grid storage would be useful to anyone with a large complement of NAS arrays to administer, according to a manager of a national Internet e-mail portal service who asked not to be named. He complains that his current complement of several hundred NAS storage devices from a prominent NAS vendor creates a huge management problem. Managing the capacity on each array requires that he access each array's self-generated status and configuration Web page, which is "like surfing the Web all day." To him, the possibility of one virtual NAS array, created from a cluster of individual arrays, is a management boon.

The development of storage grids clearly is geared toward NAS users today - primarily because NAS vendors are spearheading such efforts. But others might one day benefit from the grid storage concept, particularly those who have unruly Fibre Channel fabrics. Take for example a hospital in northern Virginia with several isolated storage-area network (SAN) islands - the result of uncoordinated storage acquisitions made by various corporate turf lords. Making disparate SANs communicate and share data with each other in the face of non-interoperable switching equipment is a nightmare for the hospital. Conceivably, by using clustered NAS devices serving as gateways and managers of the back-end SANs, the hospital would gain improved capacity, file sharing and management generally.