Wednesday, April 30, 2008

Fibre Channel - don't go far!

Graham asked me for some thoughts on a customer who wants to site Avid Unity SANs in several buildings and have clients connected over multi-mode -> single mode (and back) media translators. It sounded a bit dodgy so I went digging;

Clarion seem to be the most upfront about cable lengths for one and two gig fibre channel on their storage and they quote 500m and 300m respectively. In all the scenarios it isn't signal loss that's the issue (decent grade multi-mode cable degrades at about 0.5 dB /100m which makes the 500m for multi-mode eminently achievable by even modest splicing). The problem is all down to latency and since Avid expects so much housekeeping of a Unity to happen at the client (mirroring etc) you can see why all the Unity documentation quotes 150-180m (depending on which switch/HBA combo) for client-Unity connection. This (assuming C=2.5 x 10^8 m/s in gallium-doped OM1 glass) means the tolerable latency is around 600nS. This would not be achievable once single-mode media-translators are involved.

In a sense fibre channel is the worst thing to either send a long way and/or run through translation stacks (between it's native multi-mode to single mode and back again). It's Asynchronous but doesn't enjoy IP's fault tolerance and it's handshake'd but doesn't have TCP's dropped packed abilities. Worst of all it's time-dependent because it's video! It's a situation where playing fast'n'loose will end in tears.

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