Intel 7xx Servers, 170 Servers, AS/400 RISC Server manual Introduction

14.6.1 Introduction

In general, applications are functionally isolated from the exact nature of their storage subsystems by the operating system. An application does not have to be aware of whether its storage is contained on one type of disk or another when performing I/O. But different I/O subsystems have subtly different performance qualities, and virtual SCSI is no exception. What differences might an application observe using IBM i operating system Virtual SCSI versus directly attached storage? Broadly, we can categorize the possibilities into I/O latency and I/O bandwidth.

We define I/O response time as the time that passes between the initiation of I/O and completion as observed by the application. Latency is a very important attribute of disk I/O. Consider a program that performs 1000 random disk I/Os, one at a time. If the time to complete an average I/O is six milliseconds, the application will run no less than 6 seconds. However, if the average I/O response time is reduced to three milliseconds, the application's run time could be reduced by three seconds. Applications that are multi-threaded or use asynchronous I/O may be less sensitive to latency, but under most circumstances, less latency is better for performance.

We define I/O bandwidth as the maximum amount of data that can be read or written to storage in a unit of time. Bandwidth can be measured from a single thread or from a set of threads executing concurrently. Though many applications are more sensitive to latency than bandwidth, bandwidth is crucial for many typical operations such as backup and restore of persistent data.

Because disks are mechanical devices, they tend to be rather slow when compared to high-performance microprocessors such as IBM POWER Systems. As such, we will show that virtual hosted disk performance is comparable to directly attached storage under most workload environments.

IBM i operating system hosts disk space in a Network Storage Space (NWSSTG). A network server description (NWSD) is used to give a name to the configuration, to provide an interface for starting and stopping an AIX logical partition, and to provide a link between AIX and its virtual storage.

There are many factors that affect IBM i operating system performance in a virtual SCSI environment. This chapter discusses some of the common factors and offers guidance on how to help achieve the best possible performance. Much of the information in this chapter was obtained as a result of analysis experience within the Rochester development laboratory. Many of the performance claims are based on supporting performance measurement and analysis with a primitive disk workload. In some cases, the actual performance data is included here to reinforce the performance claims and to demonstrate capacity characteristics.

IBM i 6.1 Performance Capabilities Reference - January/April/October 2008
© Copyright IBM Corp. 2008	Chapter 14 DASD Performance	233