Planning For Arrays And Ddms; Cache - IBM TotalStorage DS6000 Series Redbooks

requirements:
HA, DR etc.
SAN Fabric
Workload from
other servers
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
per 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 356. However, the
majority of iSeries workloads do not require this.

Cache

In general, iSeries workloads do not benefit from large cache. Since there is no choice for
cache size on DS6000, you should ensure that your workload is not excessively
cache-friendly. This can be seen in OS/400 Performance Tools System, Component and
Resource Interval reports. However, in general, with large iSeries main memory sizes,
OS/400 Expert Cache can reduce the benefit of external cache.
354
DS6000 Series: Concepts and Architecture
Other
Performance
Tools Reports
Workload
statistics
Rules of thumb
Proposed
configuration
Modeling with
Disk Magic
Requirements and
expectations met ?
Yes
Finish
Workload
description
Workload
characteristics
Adjust config
based on DM
modeling
No