While FICON channels offer significant advantages in terms of channel bandwidth and performance, many users long for the old steam gauge utilization metrics provided for ESCON channels. While there are no simple rules-of-thumb for FICON channel management, this article will explore the available path utilization metrics and suggest new metrics for FICON channels.
The general availability of 2Gbit FICON channels, storage subsystems, and directors has presented performance analysts a wealth of benefits as well as a number of perplexing problems. Principal among these problems is the management of FICON channels, since none of the available metrics seem to have any relationship to the service levels achieved for the storage subsystems they support. Note that it is not our intent to provide a tutorial about the fundamentals of the FICON architecture. Rather, we will provide a detailed discussion of both available and proposed metrics managing FICON channels.
Perhaps the best way to understand this problem is to look at Figure 1, which shows the service time graph for a FICON storage subsystem. Note that a substantial jump in subsystem service time occurred just after the per-channel I/O rate exceeded 1,750 start subchannels per second (SSCHs/sec). The question is, why did this jump in service time occur?
For ESCON channels, there are reliable rules-of-thumb for channel utilization that predict and/or explain significant increases in subsystem service time. However, neither of the available metrics for FICON channels appears to provide any intuition about why the service time increased. Specifically, the two metrics—FICON channel busy and bus busy—do not seem to provide any insight into subsystem service time. While the exact definitions of FICON channel and bus busy will be discussed in greater detail in the following section, the values prior to and after subsystem saturation provide little, if any, guidance. Since the bus utilization shown in Figure 1 remained unchanged, it is reasonable to assume that it is not an effective predictor of channel saturation. While the increase in channel utilization from 30.2 to 44.5 percent initially appears to provide an appealing explanation of the substantial elongation of service time, other cases can be cited where substantially higher channel utilizations do not result in any service time problems.
Therefore, the question presented by the available FICON channel metrics is simple: What metrics can you employ to manage FICON channels?
First, we will review the circuit-switched protocol and channel program management schema used by ESCON channels. Then, we will discuss the packet switched schema employed by FICON channels.
While it is common to refer to the entire path between the processor and storage subsystem as the channel, it is important to clarify the definition. Specifically, the channel is a physical entity, which is incorporated in the processor. Whether ESCON or FICON, the channel path is the fibre between the channel and the storage subsystem, as well as the interface adapter on the subsystem and any intervening directors.