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This soft copy for use by IBM employees only.
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Rebuild Priority
This parameter sets the priority of the execution order of the rebuild I/O
requests with respect to the system I/O requests. The rebuild priority can be
changed without affecting data in the logical drive.
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Parity Placement
Parity placement defines how parity is placed in the disk array with respect
to the data. Figure 32 shows the differences between left symmetric and
right asymmetric parity placement.
AAA-LLL are the data stripe units; PP0-PP3 are the corresponding parity
units.
Right Asymmetric (RA) Left Symmetric (LS)
Disk Disk Disk Disk Disk Disk Disk Disk
1 2 3 4 1 2 3 4
PP0 AAA BBB CCC AAA BBB CCC PP0
DDD PP1 EEE FFF EEE FFF PP1 DDD
GGG HHH PP2 III III PP2 GGG HHH
JJJ KKK LLL PP3 PP3 JJJ KKK LLL
Figure 32. Parity Placement in a RAID Subsystem
The default value is right asymmetric. In some situations, you may want to
try Left Symmetric to improve performance.
Note: Once a parity placement scheme is chosen and data is stored in the
logical drives, it cannot be changed without destroying data in the logical
drives.
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Read-Ahead
Normally the IBM SCSI-2 Fast/Wide Streaming RAID adapter transfers data
from a disk to its local cache in steps of stripe unit size. This provides
excellent overall performance when workloads tend to be sequential.
However, if the workload is random and system I/O requests are smaller
than stripe unit size, reading ahead to the end of the stripe unit will result in
a wasted SCSI bus bandwidth and wasted disk utilization.
When Read-Ahead is set to Off, the size of data transfer from the disk to the
local cache is equal to the system I/O request size, and no read-ahead to the
end of the stripe unit size is performed.
Note: The Read-Ahead setting can be modified without destroying data in a
logical unit.
2.5.1.3 Defining a Hot-Spare Disk
After selecting Create/Delete Array from the main menu of the IBM RAID
Configuration program, the following panel appears:
Chapter 2. Hardware Configuration 41