Very good post by Brocade’s Dave Lytle (thank you). Thanks also to Anthony Vandewerdt for the mention.
I have been visiting with customers in Asia and Europe recently who are in the midst of doing infrastructure and/or storage refresh in their data centers. Typically the results from upgrading your fibre channel infrastructure is good news for all involved but I have found one instance where these technology refreshes might not provide the value that the customer is seeking.
FICON performance is gated by a number of factors with one of those major factors being the maximum link speed that each connection point will accept. As you know, if one end of an optical cable is 8Gbps but the other end of that cable is 4Gbps then the link is obviously auto negotiated down to 4Gbps. Usually this is not a problem, it is just a single link after all.
And if the customer (wisely) is upgrading their switched-FICON infrastructure to keep pace with their storage capabilities then our current switching products will transmit data at a maximum of either 8Gbps or 16Gbps. Our DCX 8510 16Gbps Director family is becoming very popular for technical refreshes but, of course, there are no DASD storage arrays that connect at 16Gbps – the fastest currently being 8Gbps. So those 16Gbps connections will auto negotiate down to 8Gbps or 4Gbps – the highest common link rate that both ends of the optical cable and their SFPs can provide. Of course, this is just what you would expect to have happen.
Before I touch on my concern let me lay just a little bit of groundwork.
DASD, regardless of vendor, is well known and widely used in mainframe shops. On DASD storage the typical use case is about 90% read I/O and about 10% write I/O. Every shop is different and that is not my point anyway. It is my experiences that there is still a lot of 2Gbps and 4Gbps DASD in mainframe shops but many mainframe enterprises are realizing that they need to upgrade their DASD to 8Gbps performance.
And, although it is not always architected or thought out very well, across all of the links that make up the total path between CHPIDs and storage, we should never have the target of the I/O exchange to be slower than the initiator of that I/O exchange. I will show you diagrams of what I mean very soon.
So what is my concern?
Below is a graphic representing what I consider to be a good deployment for an switched-FICON I/O infrastructure.
Read on here