Much simpler option
So have the NAS vendors been vindicated? It certainly seems that way Ã, the market is running in their direction and even the strongest supporters of SANs now acknowledge that NAS has a role in the enterprise.
As companies such as Network Appliance point out, NAS is by far the simpler option. It does not involve the installation of new networks, or many changes in the way that existing networks operate. By comparison, SANs look expensive and over-complex, requiring wholesale redesign of the systems infrastructure.
'NAS starts at the low end with a device like the Quantum Snap. It's really just a network-enabled hard disk, so it doesn't need a PC server Ã, it's self-managed and it's cheap,' said Egge. 'Conversely, a SAN means lengthy discussion of how to implement it, and what hardware and software to buy.'
One result is that SAN vendors have become increasingly conciliatory towards NAS, to the extent that the SNIA includes both sides among its membership. They are working towards a way of unifying the two Ã, or at least defining a spectrum in which they can both fit.
'The SNIA technical council took on the job of creating a layered model for storage that would take in SANs, NAS and virtual storage,' said Rickard. 'At the end of the day we agreed on more than we disagreed. It was just a question of agreeing on language.'
The SAN zoning concept is now being expanded to enable storage virtualisation, where storage is pooled and made available to the servers as pure SCSI blocks. This does away with the need to allocate physical devices to specific servers and enables dissimilar devices to be mirrored.
Virtual storage relies on there being a control device somewhere in the SAN. This is the only SAN node that sees both the storage devices and the servers as they really are Ã, everything else merely sees what it is permitted to see.
In symmetrical storage virtualisation, the control device sits astride the data path and all data requests must pass through it. But there can be multiple devices, both for redundancy and to provide improved performance. Symmetrical virtualisation is typified by devices running Datacore's SanSymphony software, such as the Gadzoox Axxess box.
Asymmetrical virtualisation places the control device alongside the data path. This allocates storage blocks to the servers, but the servers must then keep track of these allocations themselves, using modified host-bus adapters (HBAs) capable of storing the necessary block map.
The best example here is perhaps Compaq's VersaStor technology, which has attracted support from a number of other key SAN companies, including HBA developers such as Emulex, JNI and QLogic.
The symmetrical scheme has the advantage that it can work with today's SAN hardware, whereas VersaStor must await the development of new hardware. However, asymmetrical virtualisation could offer higher reliability and performance.
