See the vxtunefs man page -
http://sfdoccentral.symantec.com/sf/5.0MP3/aix/manpages/vxfs/man1m/vxtunefs.html
"read_ahead
In the absence of a specific caching advisory, the default for all VxFS read operations is to perform sequential read ahead. The enhanced read ahead functionality implements an algorithm that allows read aheads to detect more elaborate patterns (such as increasing or decreasing read offsets, or multithreaded file accesses) in addition to simple sequential reads. You can specify the following values for read_ahead:
0
Disables read ahead functionality
1
Retains traditional sequential read ahead behavior
2
Enables enhanced read ahead for all reads
By default, read_ahead is set to 1, that is, VxFS detects only sequential patterns.
read_ahead detects patterns on a per-thread basis, up to a maximum of vx_era_nthreads. The default number of threads is 5, however, you can change the default value by setting the vx_era_nthreads parameter in the system configuration file, /etc/system."
