Sunday 23 July 2006

SAN and NAS - basically shared storage

Shared storage here refers to computer storage or hard disks.

Anyone doing film effects or film finishing knows that as the number of people working together on a project increases, copying from one computer to another is no longer very convenient. So one needs to keep data in one place and share it among different users accross the network.

There two kinds of shared data arrangements in use now - SAN and NAS. Plenty of definitions exist for these terms. I like to think of it like this. SAN is a network of storage and users. NAS is a storage attached to an existing network.

SANs consist of computers with high speed fibre channel cards installed. These connect to a fibre channel switch. The shared storage - SAN - ia also connected to this switch. And there is a server computer also attached to this switch that just directs what goes where. Hence, Storage Area Network - SAN.

This 'switch' is like a hub interconnecting on demand, all computers connected to it. And transmitting data from whoever asks for it, to who needs it.

SANs allow extremely high speeds of data to be transferred simultaneously accross computers connected to the switch. It is possible to have a SAN that can allow up to 4 computers play full reesolution film sequences (each about 300 MBytes/sec). On the flip side SANs are problematic to upgrade either in capacity or the number of users. And even the number of users handled needs to be small - like 4-8. And, of course, SANs are also very expensive.

NASs on the other hand consist of a bunch of disks attached to server computer which is attached to a normal Ethernet network switch. A normal Gigait switch. The switch also connects to all computers around. Any computer needing data accesses the server which fetches the data and hands it out.

Expanding a NAS is comparatively easy with disks just being added. Expanding the users is as simple as plugging the user into the Ethernet switch. NAS boxes are comparatively cheap. Some are so economical that they just connect to the network and one can attach any drive to them. Like a converter.

But NAS is not as fast because it works on a network at network speeds. And is subject to network conditions. Even with GigE, on a busy network you get speeds no better than 60 MB/sec. Means 1 Gb copied in 15 sec. With some switches under some conditions you can even go up to 80 or 90 MB/sec. But no more than that. Surely a problem for anything over standard video resolution.

The interconnecting electronics in a SAN is Fibre Channel. At the current state, each Fibre Channel can handle 4 Gbps - or about 500 MBytes/sec max. With multiple simultaneous channels, this figure increases.

The interconnecting electronics in a NAS is Ethernet. At the current state, each Ethernet port can handle 1 Gbps - or about 125 MBytes/sec max. In less than a year, there will be 10 GigE which means each Ethernet port will handle 1250 MBytes/sec.

Who knows, by then, NAS speeds would equal or better today's SAN speeds.