When you invest in new mainframe disk or tape storage equipment, you’re most likely looking at FICON equipment. Most modern disk and tape sub- systems are so fast they need FICON channels to exploit the performance potential they offer. FICON has been around for five years and all vendors are in at least their second iteration of the hardware, software, and firmware. This means you can expect a smooth, fast ride when investing in FICON.
The FICON protocol also has many inherent advantages over the ESCON protocol previously used. FICON supports distances well over 10 km with hardly any speed degradation or data droop. As FICON supports higher effective data rates per link, you’ll need fewer links, so it will be more cost-effective to buy a FICON solution. Of course, with a substantial investment in ESCON technology for your installation, you’ll need to make and cost-justify the transition.
This article examines how you can determine the number of FICON channels that will be required, based on the workload in our example installation. We won’t discuss the operational challenges of performing this upgrade in your shop.
First, let’s recap some ESCON planning observations.
ESCON Configuration Planning
As you’re likely used to planning ESCON configurations, it’s useful to review some of the ESCON configuration rules and explicitly consider how they influenced the design of current channel and ESCON director configurations. The maximum data rate for each ESCON channel is about 18 Mbyte/ second during a data transfer peak. While this data rate can be achieved for an individual job such as a backup or restore, it’s clearly impossible to reach such data rates during your typical peak hour. Most installations will plan for peak channel utilization of less than 60 percent, resulting in an absolute maximum effective data rate of about 10 Mbyte/s during the peak period. With higher channel utilizations, pending and other queuing delays would slow down the system too much.
There’s also a limit to the number of I/O operations an ESCON channel can handle, as the channel is “connected” during interpretation of the I/O action. The connect time for a simple I/O such as a 4Kb read-hit can range from 0.5 to 1 ms, mostly depending on the disk subsystem year of manufacture. For a typical workload, with larger blocks and cache misses, too, this means the number of I/O operations per channel will be well below 1,000 I/O operations per second.
Finally, ESCON has inherent addressing limitations in the protocol. ESCON can address up to 1,024 different devices per channel. For a disk subsystem, this means a set of channels is required for every four Logical Control Units (LCUs) with 256 addresses each. It also means only 3TB can be addressed when using 1,024 volumes with 3390 Model 3 format. Modern controllers support 16 or more LCUs, so the number of required ESCON channels will grow proportionally.
The FICON protocol differs greatly from the ESCON protocol. The theoretical maximum data rate is much higher at 200 Mbyte/s in each direction for the 2Gbs FICON Express channel. The protocol also allows multiple I/O operations to be handled simultaneously on a channel as packets can be interleaved. Of course, when multiple operations are running concurrently, they share the bandwidth.