IBM DS8000 manual Planning for arrays and DDMs, Cache

Performance		Workload
Tools Reports		description
Workload		Workload
statistics	characteristics
Other
requirements:
HA, DR etc.
	Rules of thumb
SAN Fabric
	Proposed
	configuration
Workload from
other servers
	Modeling with	Adjust config
	Disk Magic
		based on DM
		modeling
Requirements and
expectations met ?		No
	Yes
	Finish

Figure B-34 Process for sizing external storage

Planning for arrays and DDMs

In general, although it is possible to use 146 GB and 300 GB 10K RPM DDMs, we recommend that you use 73 GB 15K RPM DDMs for iSeries production workloads. The larger, slower drives may be suitable for less I/O intensive work, or for those workloads which do not require critical response times (for example, archived data or data which is high in volume but low in use such as scanned images).

For workloads with critical response times, you may not want to use all the capacity in an array. For 73 GB DDMs you may plan to use about 300 GB capacity per 8 drive array. The remaining capacity could possibly be used for infrequently accessed data. For example, you may have archive data, or some data such as images, which is not accessed regularly, or perhaps FlashCopy target volumes which could use this capacity, but not impact on the I/O /sec on those arrays.

For very high write environments, you may also consider using RAID-10, which offers a higher I/O rate per GB than RAID-5 as shown in Figure B-3 on page 399. However, the majority of iSeries workloads do not require this.

Cache

In general, iSeries workloads do not benefit from large cache, so in the initial planning for Disk Magic, consider the minimum cache size available. Depending on the workload (as shown in OS/400 Performance Tools System, Component and Resource Interval reports) you may see

Appendix B. Using DS8000 with iSeries 397