HP StorageWorks HSG80 ACS Solution Software V8.8 for HP-UX Installation and Configuration Guide (AA-RV1FA-TE, March 2005)

Planning Storage Configurations
96 HSG80 ACS Solution Software V8.8 for HP-UX 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, separate I/O requests can be handled
in parallel, which increases the request rate. This concept is shown in Figure 32.
Figure 32: Large chunk size increases request rate
Large chunk sizes also tend to increase the performance of random reads and
writes. HP 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 your subsystem, you must first
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, 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