IBM SG24-4763-00 manual Defining a Hot-Spare Disk, Parity Placement in a RAID Subsystem

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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.

•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)

Disk Disk Disk Disk 1 2 3 4

PP0 AAA BBB CCC

DDDPP1 EEE FFF GGG HHH PP2 III JJJ KKK LLL PP3

Left Symmetric (LS)

Disk Disk Disk Disk 1 2 3 4

AAABBB CCC PP0 EEE FFF PP1 DDD III PP2 GGG HHH 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.

•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. H a r d w a r e Configuration 41