IBM manual How does the DS8000 address the challenge?, FC-AL shortcomings, 257

relatively small logical volumes, we ran out of device numbers to address an entire LSS. This happens even earlier when configuring not only real devices (3390B) within an LSS, but also alias devices (3390A) within an LSS in z/OS environments. By the way, an LSS is congruent to an logical control unit (LCU) in this context. An LCU is only relevant in z/OS and the term is not used for open systems operating systems.

12.3 How does the DS8000 address the challenge?

The DS8000 overcomes the architectural limits and bottlenecks which developed over time in the ESS due to the increasing number of I/Os and MB/sec.

In this section we go through the different layers and discuss how they have changed to address performance in terms of throughput and I/O rates.

12.3.1 Fibre Channel switched disk interconnection at the back end

Because SSA connectivity has not been further enhanced to increase the connectivity speed beyond 40MB/sec, Fibre Channel connected disks were chosen for the DS8000 back end. This technology is commonly used to connect a group of disks in a daisy-chained fashion in a Fibre Channel Arbitrated Loop (FC-AL).

FC-AL shortcomings

There are some shortcomings with plain FC-AL. The most obvious ones are:

￿As the term arbitration implies, each individual disk within an FC-AL loop competes with the other disks to get on the loop because the loop supports only one operation at a time.

￿Another challenge which is not adequately solved is the handling of failures within the FC-AL loop, particularly with intermittently failing components on the loops and disks.

￿A third issue with conventional FC-AL is the increasing time it takes to complete a loop operation as the number of devices increases in the loop.

For highly parallel operations, concurrent reads and writes with various transfer sizes, this impacts the total effective bandwidth of an FC-AL structure.

How the DS8000 series overcomes FC-AL shortcomings

The DS8000 uses the same Fibre Channel drives as used in conventional FC-AL based storage systems. To overcome the arbitration issue within FC-AL, the architecture is enhanced by adding a switch-based approach and creating FC-AL switched loops, as shown in Figure 12-2 on page 258. Actually it is called a Fibre Channel switched disk subsystem.

These switches use FC-AL protocol and attach FC-AL drives through a point-to-point connection. The arbitration message of a drive is captured in the switch, processed and propagated back to the drive, without routing it through all the other drives in the loop.

Chapter 12. Performance considerations 257