RAID Array 450 HSZ50 HSOF Software Version 5.7 for Compaq Tru64 UNIX Release Notes

Release Notes
AA–R1ZZE − TE Page 41
Large chunk sizes also tend to increase the performance of random reads and
writes. It is recommended that you use a chunk size of 10 to 20 times the
average request size, rounded to the closest prime number. In general, a chunk
size of 239 works well for Compaq Tru64 UNIX systems with a transfer size of
16 sectors; 113 works well for OpenVMS systems with a transfer size of 8
sectors.
To calculate the chunk size that should be used for your subsystem, you first
must analyze the types of requests that are being made to the subsystem:
• Lots of parallel I/O 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 don’t know, then you should use a chunk size of 15 times the
average transfer request rate.
• If you have mostly sequential reads or writes (like those needed to work
with large graphic files), then make the chunk size a small number (i.e. 17
sectors in Table 8-1 shows a few examples of chunk size selection.
Table 8-1 Example Chunk Size
Transfer
Size (KB)
High
Locality
Unknown
Locality
Low
Locality
2 41 59 790
4 79 113 163
8 157 239 317
8.1.1.3 Increasing the Data Transfer Rate
A small chunk size relative to the average request size increases the data
transfer rate by allowing multiple disk drives to participate in one I/O request.
This concept is shown in Figure 8–2.