Compaq StorageWorks HSG60 ACS Solution Software V8.6 for Compaq Tru64 UNIX Installation and Configuration Guide

2–24 HSG60 ACS Solution Software Version 8.6 for Compaq Tru64 UNIX Installation and Configuration Guide
Increasing the Request Rate
A large chunk size (relative to the average request size) increases the request rate by
enabling multiple disk drives to respond to multiple requests. If one disk drive contains
all of the data for one request, then the other disk drives in the storageset are available
to handle other requests. Thus, in principle, separate I/O requests can be handled in
parallel, thereby increasing the request rate. This concept is shown in Figure 2–15.
Figure 2–15. Chunk size larger than the request size
Large chunk sizes also tend to increase the performance of random reads and writes.
Compaq recommends that you use a chunk size of 10 to 20 times the average request
size, rounded to the closest prime number.
To calculate the chunk size that should be used for the subsystem, you first must
analyze the types of requests that are being made to the subsystem:
■ Many parallel I/Os that use a small area of disk should use a chunk size of 10 times
the average transfer request rate.
■ Random I/Os that are scattered over all the areas of the disks should use a chunk
size of 20 times the average transfer request rate.
■ If you do not know, then you should use a chunk size of 15 times the average
transfer request rate.
CXO-5135A-MC
Chunk size = 128k (256 blocks)
Request A
Request B
Request C
Request D