Understanding Subscription Ratios on Cisco FC Modules
It was always time consuming for me to decipher Cisco documentation that related to port group bandwidth and module overscription rates for their Gen-2 FC Modules. So what follows is a brief interpretation of how those ratios are determined and why they matter. Currently the modules available are 12 port, 24 port, 48 port and the 4 port 10G cards. The 12 port card offers a full 4G line rate per port, the 24 and 48 port cards share their bandwidth and operate according to the following subscription ratio’s:
So what this means is that if all ports were active on a 24 port module they would operate at the full line rate of 2Gbps. Broken down this equates to four 6 port port groups, each group capable of 12.8 Gbps. The 48 port modules consist of four 12 port port groups, each group again capable of 12.8 Gbps. That being said the full line rate of each port on a 48 port module if all ports were active would be 1Gbps. The maximum bandwidth per port per module, minus the 10G mod, is 4Gbps. So why does all this matter? Well its extremely important for host placement on those fiber switches. Ideally if you had a pleothra of modules in your director, you could seperate your traffic as follows:
12-port 1/2/4 FC Module-high perfomance servers, front end array ports, ISL’s, and tape drives/libraries
24-port 1/2/4 FC Module-server connections, front end array ports
48-port 1/2/4 FC Module-tier storage solutions (low,medium and high), server connections
4-port 10Gbps FC Module-ISL consolidation, high performance metro connections
These recommendations are highly dependent on your environment as there is nothing preventing you from putting a front end array port on a 48 port card. But being aware of what other devices exist on that module can save you trouble in the long run, not to mention it will ease you down the path of scalability. In addition, you should not only be cognizant of per module placement but also per port group placement. Remember you only have 12.8Gbps of bandwidth per port group so loading up all you front end ports, high performance servers, etc. into one port group is a recipe for a poor design.
