IBM TotalStorage DS6000 Series: Performance Monitoring and Tuning

Chapter 11. iSeries servers 389

򐂰 Switchable: Switched between two systems or partitions in a clustered environment

This is very different from the way auxiliary storage (disk) was regarded prior to OS/400

V5R1. Until then, all iSeries disks were considered to be owned and usable only by a single

system. Enhancements made in this and later releases make using independent disk pools

an attractive option for many customers who are looking for higher levels of availability and

server consolidation. We use the terms

independent disk pool and independent auxiliary

storage pool

interchangeably.

There are three types of auxiliary storage pools (ASP):

򐂰 System auxiliary storage pool (ASP 1): This storage pool contains OS/400 and licensed

program products, plus any user objects.

򐂰 Basic user auxiliary storage pool (ASP 2-32): Prior to OS/400 V5R2, ASPs 2-32 were

known as user storage pools. Their function has not changed, but they are now referred to

as basic user ASPs. They allow the disk storage attached to a single iSeries server to be

grouped into separate pools. However, these pools have a close relationship to the system

ASP.

򐂰 Independent auxiliary storage pool (ASP 33-255): This disk pool type contains objects,

directories, or libraries that contain the objects, and other object attributes such as

authorization and ownership attributes. An independent disk pool can be made available

(varied on) and made unavailable (varied off) to the server without restarting the system.

When an independent auxiliary storage pool is associated with a switchable hardware group,

it becomes a switchable auxiliary storage pool and can be switched between one iSeries

server and another iSeries server in a clustered environment. Note that with the required

hardware, internal iSeries disks can be switchable. External storage servers are not required

in order to switch storage from one iSeries server to another. Achieving the full benefits of an

external storage server in an iSeries environment requires the storage server be set up as its

own independent ASP.

11.1.4 Internal versus external storage on iSeries

Disk volumes can either be internal to the iSeries or attached externally. Disk volumes are

grouped in auxiliary pools (ASPs). Disk volumes can be protected or unprotected. Protection

for internal disk volumes can be OS/400 software mirroring or RAID 5 hardware protection.

When the iSeries disks are external, as when using a DS6000, the disk devices are mapped

into the Logical Unit Numbers (LUNs) that are carved in the DS6000 Ranks. In the DS6000

LUNs are striped across a Rank, with the Ranks being either RAID 5 or RAID 10 protected.

The DS6000 can accommodate all iSeries disks, including the load source unit. Load source

units on external storage devices are supported only on

~ i5 models and on i5/OS

V5R3 or later.

Since iSeries servers already make use of cache in main storage, iSeries workloads

generally do not benefit from large cache in the DS6000 as much as other server platforms

do. Also, the large iSeries cache means that its sequential reads from DS6000 do not follow

the pattern of sequential reads typical of other server platforms, so the DS6000 Sequential

Adaptive Replacement Cache (SARC) algorithms may not provide the benefit in an iSeries

environment that they do on other server platforms.

Although iSeries performs well with internal storage, it also performs well with external

storage, providing clients the flexibility of separating server management from storage

management. In addition, external storage seamlessly becomes part of the iSeries single

level storage environment.