Do you have any recommendations, dos and don'ts, pitfalls, risks? Is there anyway to obtain information on others' experiences with channel extension used in this manner?
Please be advised that moving tape backup streams over extended channels would require substantial bandwidth between sites A and B. I'm not sure if you currently have dedicated dark fiber between sites, but that may be the minimum that would be required for multiple concurrent backup streams. 250 miles is also very far for extended channels. 100KM to 200KM is usually the top-end distance for either Fibre Channel (FC) or FICON extension before latency or "droop" occurs (ESCON extension is MUCH less tolerant to distance).
200KM is about 124 miles as the crow flies, so even using dark fiber connections and great extension gear will only get you halfway there for channel extension. Using an IP bridge to your fabric with a leased IP connection would be a better solution for that type of distance. Buffering, packet shaping, compression and spoofing of the connection can reduce a lot of the latency issues. Common Mainframe PPRC, XRC and GDPS configurations can utilize connections such as this. An even better approach may be to use SONET as the connection medium, since some of the FICON to SONET vendors support high-bandwidth trunking, and distances up to 40,000KM.
Your bandwidth requirements will depend on how much data you need to move across the links. I use a simple formula of 10 Mbit of IP bandwidth required for every 1 megabyte (MB) of data per second transmitted. As for do's, don'ts, pitfalls, and risks, there are too many to discuss using this as a venue. You should sit down with your storage, tape and switch vendors and have an in-depth planning session to go over all the details.
The tape vendors you mentioned in your question have a lot of experience in this area, and may be a very useful resource for you, as would your FICON switch vendor. You did not mention the storage vendor you are using, but I would also include them on the planning session.
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