Intel AS/400 RISC Server, 170 Servers, 7xx Servers manual Disk I/O CPU Cost

	For Each Target HBA	For Each NWSD
Machine Pool:	21 MBytes	1	MByte
Base Pool:	1 MByte	0.5		MByte
QFPHIS Private Pool:	0.5 MByte	1	MByte12
Total:	22.5 MBytes	2.5		MBytes

Warning: To ensure expected performance and continuing machine operation, it is critical to allocate sufficient memory to support all of the devices that are varied on. Inadequate memory pools can cause unexpected machine operation.

17.4 Disk I/O CPU Cost

Disk Operation Rules of Thumb	CPWs13 / 1k ops/sec
iSCSI linked disks	190
IXS/IXA static or dynamically linked disks with write caching	130
enabled
IXS/IXA shared or quorum linked disks or write caching	155
disabled

While the disk I/O activity driven by the IXS/IXA or iSCSI is not strictly a “CPW” type load, the CPW estimate is still a useful metric to estimate the amount of i5/OS CPU required for a load. You can use the values above to estimate the CPW requirements if you know the expected I/O rate. For example, if you expect the Windows application server to generate 800 disk ops/sec on a dynamically or statically linked storage space, you can estimate the CPW usage as:

130 cpws/1kops * 800ops * 1kops/1000ops = 130 * 800/1000 = 104 CPWs

out of the host processor CPW capacity. While it is always better to project the performance of an application from measurements based on that same application, it is not always possible. This calculation technique gives a relative estimate of performance.

These rules of thumb are estimated from the results of performing file serving or application types of loads. In more detail, the chart below indicates an approximate amount of host processor (in CPW) required to perform a constant number of disk operations (1000) of various sizes. You can reasonably adjust this estimate linearly for your expected I/O level.

12Private pool assigned to QFPAIS must still be a 4 MB minimum size.

13A CPW is the “Relative System Performance Metric” from Appendix C. Note that the I/O CPU capacities may not scale exactly by rated system CPW, as the disk I/O doesn’t represent a CPW type of load . This calculation is a convenient metric to size the load impacts. The measured CPW cost will actually decrease from the above values as the number of processors in the NWSD hosting partition increases, and may be higher than estimated when partial processors are used.

IBM i 6.1 Performance Capabilities Reference - January/April/October 2008
© Copyright IBM Corp. 2008	Chapter 17 - Integrated BladeCenter and System x Performance	286