SAN versus NAS

The big divide in the storage world is NAS and SAN, so how do you choose between them?

NAS – network attached storage is the obvious way to do things and it is what you end up with if you "grow" a network. You take a server, install a number of disk drives which usually you then initialise into a RAID array to provide some protection against errors. The storage so provided is then shared across the network in the usual way. This works reasonably well when you need to provide a medium sized store that fundamentally doesn’t change too much in structure over time.

A SAN, on the other hand, takes a radical step outside this approach and builds a server that looks like a disk drive to other servers. To clarify, a SAN is a sort of virtual disk drive that other servers can use to provide NAS-like storage. This may sound strange but the idea has lots of advantages.

The SAN can have lots of hard drives arranged to provide a "pool" of storage from which any number of logical volumes can be constructed. When a client connects to a SAN it can ask for a block of storage to be allocated from the pool and it can then partition, format and share it as if the block of storage was "real". This provides flexibility in provisioning. In addition, the drives used to provide the pool can be added to make the storage pool bigger and failed drives can be replaced without disturbing the client's view of the storage. Notice that it is the SAN that provides the fault tolerance.

The only real problem with this approach is finding a way to connect the SAN to its client NAS servers which is fast enough. Until recently the only way to do this was to make use of a dedicated high speed fibre LAN to connect the SAN to its clients. Today we have the option of using low cost Gigabyte network connections as well. As a result the applicability of the SAN approach has migrated down to more modest storage situations.

The SATABoy reviewed in this month's storage lab provides 14TBytes in an easy to use rackmount box at an affordable price.