Q: Do I really need Xsan?

AFP is easy if you just need to share files, but won't be enough by itself if you need to stream video or audio. Assuming you know how to build one, an Xsan would require three times the equipment and at least a few thousand dollars in licenses.

If you need affordable, high-bandwidth AFP, a popular solution is to share a fast SAS RAID array attached to an Xserve or Mac Pro over a multi-port ethernet card with a link-aggregated switch. That will give you about 70-100 MB/s to four or five workstations.

JM

Link Aggregation refers to the ethernet card and switch. The goal, if you are sharing over ethernet, it to create the fastest connection you can to the switch, so your pool of users aren't limited by the bandwidth of a single GigE connection. Link aggregating several GigE connections can give you a total bandwidth to the switch of 5-600 MB/s which your users can then all share from.

The next bottleneck is the connection from your storage to your server. 4Gb Fibre is good for about 380 MB/s, or the bandwidth required to saturate about 3-4 GigE connections. A better solution would be to use either an 8Gb Fibre array (~750MB/s) or a good SAS-attached array (~800 MB/s), both of which will saturate 6-8 GigE connections.

Fibre doesn't work in the same way that ethernet does. Combining multiple fibre connections (called multi-pathing) requires either a filesystem that supports it natively (i.e., Xsan) or firmware enabled card + driver combinations offered by specific vendors for use with their products (i.e., SNS Eclipse cards used with an Evo).

Depending on what you are trying to share (video, audio, word documents?) you can get more or less extreme with your solution. Video is difficult because the file sizes are so large, and the editors are expecting whatever you implement to have the smooth playback of locally attached storage. But, if you are dealing with VFX, or motion graphics, the file sizes are still large, but the streaming requirements go way down. So it depends on what you need to accomplish.

As for Xsan, with 4-8 RAID chassis, and 8Gb (or dual 4Gb) fibre, you can get 750 MB/s per workstation. You could implement an Xsan with your single Promise, but you would be limited to the bandwidth of the single array or about 700 MB/s total shared among however many clients you have. But, even to do that you'll need at least one server in place as a dedicated metadata controller, a Qlogic fibre switch, two GigE ethernet switches, plus an Apple/LSI fibre card and Xsan license for every machine. The situation gets even more complex if you add PCs, as then you need StorNext FX licenses to connect them as clients, which are even more expensive. You can add non-fibre connected clients using an (additional) server to re-share the Xsan volume over AFP, but the re-sharing becomes more difficult if you must also have fibre PC clients attached, so for best performance it's often best to ommit one or the other.

Really though, an Xsan requires quite a bit of knowledge to successfully install and administrate. It seems a good, fast array shared over ethernet through link aggregation would be the easiest and most affordable solution for you. If your need is for post-production, a Studio Network Solutions Evo server would be a good option as well.

JM