Tuning the number of SCSI disk request blocks

Performance Guide
Chapter 5, Tuning I/O resources

The sar -S command (or its equivalent mpsar -S for SMP) reports the usage of SCSI request blocks:

23:59:44 reqblk/s oreqblk/s 23:59:49 12.56 0.00 23:59:54 3.04 0.00 23:59:59 25.45 0.00

Average 13.68 0.00

reqblk/s is the average number of request blocks used per second. oreqblk/s is the number of times per second that processes were put to sleep because not enough request blocks were available. If oreqblk/s is greater than zero, increase the value of SDSKOUT by at least the maximum value reported for the following quantity:

SDSKOUT * oreqblk/s / reqblk/s

The following table summarizes the commands that you can use to view SCSI request block activity:

Table 5-4 Viewing SCSI request block activity

 ---------------------------------------------------------------------
 Command      Field        Description
 ---------------------------------------------------------------------
 [mp]sar -S   reqblk/s     number of SCSI request blocks allocated per
                           second
              oreqblk/s    number of times per second that the system
                           ran out of SCSI request blocks