A primary objective of ESCON, upon its introduction in 1990, was to simplify configuration management. At that time, several hundred MIPS and, at most, a few terabytes of storage typified even the largest MVS configurations. Before ESCON, 4.5MB/sec. copper bus and tag cables connected these resources. The cables weighed about onehalf pound per linear foot and maximum lengths were limited to 400 feet.
In contrast to parallel channel technology, ESCON cables were comprised of two lightweight optical fibres; they supported data transfer rates and maximum transfer distances of 20MB/sec. and 9Km, respectively. Moreover, ESCON directors introduced switching and eliminated the requirement of a point-to-point connection between every processor and storage resource.
Over the next decade, the exploitation of these technologies resulted in fundamental changes in the design of storage configurations and data centers. These changes included the:
- Introduction of Parallel Sysplex technology to simplify the management of complex configurations
- Growth in the maximum size of a storage subsystem from 180 gigabytes to multiple terabytes
- Introduction of remote copy schemas for disaster recovery and replication. While 256 channels seemed ample when ESCON was introduced, this limit was soon reached.
Although every Sysplex is unique, those who have encountered or are approaching the 256 ESCON channel limit all share some simple characteristics. Specifically, these sites employ 96 to 160 channels on each processor in the Sysplex for DASD, with the remaining channels being assigned to tapes, printers, channel-to-channel adapters, and other functions. Since eight-channel sets typically support DASD devices, this yields 12 to 20 sets, each of which can support 1,024 logical devices. Usually, installations have employed most of the available device addresses to define 3390 logical volumes.
If we assume a mix of 3390 -3s and -9s behind each set of channels, we find that these parameters result in aggregate maximum storage capacities for a Sysplex ranging from 20 to 70 terabytes for the channel configurations. Moreover, the larger the storage configuration for a given number of ESCON channels, the higher the expected minimum response time due to aggregate bandwidth limitations. While individual Sysplexes at or near the 256-channel limit may be somewhat smaller or larger, the maximum practical storage capacity of a Sysplex was capped at a value far less than 100 terabytes. This limit is the box (i.e., z/ Box) that large ESCON Sysplexes have been stuck in for the last several years!
To understand the complexity imposed by ESCON, consider the configuration shown in Figure 1. It depicts eight ESCON channels from a single processor connected to two storage subsystems that support 1,024 logical volumes (addresses 3000-33FF) via two ESCON directors. Each line between the processor, directors, and storage subsystems represents four ESCON channels. This presents a microcosm of the Sysplex. A typical Sysplex might have a half-dozen processors, multiple storage subsystems, and one to four ESCON director pairs.
The eight channels depicted are connected (daisy chained) to two storage subsystems, each supporting two Logical Control Units (LCUs). Each of the LCUs supports 256 logical volumes for a total of 1,024 logical volumes behind the eight-channel set. Often, there are four daisy-chained storage subsystems behind each set of channels to define the logical volumes. Hence, z/Box defines several significant complexity issues along with aggregate storage size limits we’ve already discussed. They include the number of:
- Volumes required to define the storage — Defining a 60 to 70 terabyte storage configuration requires approximately 15,000 to 20,000 logical volumes, depending on the mix of 3390-3s and -9s. You should tune these volumes and must back them up periodically for disaster recovery purposes.
- LCUs — Assuming 256 logical volumes per LCU, 80 LCUs would have to be defined and managed.
- Storage subsystems — Assuming a mix of eight and 16 ESCON channel-capable subsystems, 10 to 20 storage subsystems would be required.
- ESCON cables — For the typical complex Sysplex, thousands would be required to connect the processors, directors, and storage subsystems.
Even if we ignore operational and tuning issues, the physical inventory problem presented to the storage manager is almost overwhelming. Moreover, if a remote copy schema is employed, a session (copy instance) must be defined for each logical volume, additional physical connectivity must be provided for each subsystem, and the entire configuration must be replicated at a second site!