Type FC channels scale very well. In other words, n channels can permit an almost n-fold throughput. A prerequisite for this is that the BS2000 CPUs and the peripherals are sufficiently powerful.
Special features for /390 servers
With SE servers SE710 / SE730, Fibre Channels with a data rate of 16 Gbit/s are supported. The following can be ascertained from the measurements in section "Measurements for storage systems":
The maximum channel throughputs are achieved only in the case of high parallelism, the throughput of each individual task being considerably lower than without a competitive load. An individual task alone can already claim 200-300 Mbyte/s. If, for example, three or four backup jobs are being performed over the same channel, they already constrain each other, even though their accumulated throughput is still lower than the measured maximum throughput of the channel. This also concerns the IO rates in the case of transaction-oriented loads.
This results in the following recommendations:
For important productive loads and time-critical batch loads, the IO requirements should be known. If a number of IO-intensive tasks are operating simultaneously, a correspondingly higher number of channels should be provided.
When multiple channels with a low utilization level are used, parallel loads can be handled better than with a few heavily used channels, even if no overload exists in the latter case.
Parallel IOs to the same volume can be expedited by PAV. If the software IO times measured with openSM2 are significantly higher than the hardware IO times, the use of PAV should be considered (see section "PAV (Parallel Access Volume) on /390 servers"). This increases the channel utilization.
When migrating to a new channel technology, one should therefore consider:
- The 16-Gbit technology provides double throughput for parallel, throughput oriented loads. However, especially for transaction oriented loads with small IOs, the gain is much lower, so that in this case, the number of channels must not simply be reduced by half!
Special features for x86 servers
A function which is similar to PAV is provided for x86 servers in the form of the function Remote System Call (RSC) for disks. This permits up to six simultaneous I/O operations to/from a volume or device.
Serialization takes place in the storage system. RSC is used by default by the BS2000 operating system components; the user does not need to specify any special setting in the configuration to use RSC or to permit parallel I/O operations.