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During an attempt to balance the load within the storage unit, placement of
application data is the determining factor. The following resources are the most
important to balance, roughly in order of importance:
v
Activity to the RAID disk groups. Use as many RAID disk groups as possible
for the critical applications. Most performance bottlenecks occur because a few
disks are overloaded. Spreading an application across multiple RAID disk
groups ensures that as many disk drives as possible are available. This is
extremely important for open-system environments where cache-hit ratios are
usually low.
v Activity to the clusters. When selecting RAID disk groups for a critical
application, spread them across separate clusters. Because each cluster has
separate memory buses and cache memory, this maximizes the use of those
resources.
v Activity to the device adapters. When selecting RAID disk groups within a
cluster for a critical application, spread them across separate device adapters.
v
Activity to the SCSI or Fibre Channel ports. Use the IBM System Storage
Multipath Subsystem Device Driver (SDD) or similar software for other
platforms to balance I/O activity across SCSI or Fibre Channel ports.
Note: For information about SDD, see IBM System Storage Multipath Subsystem
Device Driver User's Guide . This document also describes the product
engineering tool, the ESSUTIL tool, which is supported in the pcmpath
commands and the datapath commands.
Storage consolidation
When you use a storage unit, you can consolidate data and workloads from
different types of independent hosts into a single shared resource.
You might mix production and test servers in an open systems environment or mix
open systems, System z and S/390 hosts. In this type of environment, servers
rarely, if ever, contend for the same resource.
Although sharing resources in the storage unit has advantages for storage
administration and resource sharing, there are additional implications for workload
planning. The benefit of sharing is that a larger resource pool (for example, disk
drives or cache) is available for critical applications. However, you must ensure
that uncontrolled or unpredictable applications do not interfere with critical work.
This requires the same kind of workload planning that you use when you mix
various types of work on a server.
If your workload is critical, consider isolating it from other workloads. To isolate
the workloads, place the data as follows:
v
On separate RAID disk groups. Data for open systems, System z or S/390 hosts
are automatically placed on separate arrays, which reduces the contention for
disk use.
v On separate device adapters.
v In separate storage unit clusters, which isolates use of memory buses,
microprocessors, and cache resources. Before you make this decision, verify that
the isolation of your data to a single cluster provides adequate data access
performance for your application.
Chapter 2. Hardware and features
35