A SAN is a storage area network. A NAS device is network-attached storage. A SAN serves blocks to servers accessing dedicated areas of storage. A NAS device serves files to servers accessing shared storage. An iSCSI storage 'thing' serves blocks to servers accessing dedicated areas of storage. So it must be a SAN, mustn't it?
Let's try comparing an iSCSI 'thing' with a printer linked to servers and PCs by Ethernet. We don't say that the printer is in a PAN, a printer area network. Similarly I don't think we should view an iSCSI storage 'thing' as being, logically, in a SAN.
Merely because there is an Ethernet line to an iSCSI storage device controller, behind which are drive arrays, or tape devices, doesn't make it a storage area network. It is much more like, to my mind, the network-attached storage model.
We would, correctly, refer to the printer linked to servers by Ethernet, as a network-attached printer.
NetApp has becomes the largest vendor of iSCSI 'SANs' by changing its filers to serving blocks and sticking iSCSI target interfaces on them. Voila, what was once a NAS box, a filer, becomes a SAN.
It's not difficult to build iSCSI SANs in this way. Sure you can build complex ones, ones with Fibre Channel access as well as iSCSI, ones with endlessly scalable, fully redundant, load-blanced storage controllers and storage - think Pillar Data. But you can also build simpler mid-range iSCSI block-level network-attached storage for mid-range and smaller enterprises.
Think Intransa, EqualLogic, Lefthand Networks, DataCore with commodity hardware. Think Microsoft, soon, very soon. Think EMC. Think Sun DMG. Surely everyone will be doing it.
Because it is technically easy. Because 10 gigabit Ethernet is coming.
The old iSCSI SAN-in-the-enterprise model was as a way of extending Fibre Channel SAN access to non-FC servers. You put iSCSI target devices into the FC fabric and voila, iSCSI access.
The new way is to forget the FC fabric altogether. It's network-attached storage, at the block level, for the great FC SAN unwashed, the hundreds of thousands of medium and small enterprises.
Do they want network-attached block storage? Is it better than network-attached file storage for them?
I don't know if there is a clear answer to that question. But I suspect one attribute of network-attached iSCSI storage (NAIS) will be better drive utilisation from block-level efficiencies.