Intel AS/400 RISC Server - Manual (Page 233)

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
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Copyright IBM Corp. 2008 Chapter 14 DASD Performance 233