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PC Server 500 - Using the advanced functions
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Using the Advanced Functions
This section gives the procedures for using the advanced functions, such as changing the write policy, changing the RAID parameters, and formatting a drive.

Warnings appear throughout this section to alert you to potential loss of data and should be heeded before answering yes to the confirmations requested by the RAID configuration program.

Changing the Write Policy
When you configure a logical drive, the RAID adapter automatically sets the write policy to write-through (WT) mode, where the completion status is sent after the data is written to the hard disk drive. To improve performance, you can change this write policy to write-back (WB) mode, where the completion status is sent after the data is copied to cache memory, but before the data is actually written to the storage device.

Although you gain performance with write-back mode, it creates a greater risk of losing data due to a power failure. This is because the system gets a completion status message when the data reaches cache memory, but before data is actually written to the storage device.

To change the write policy:
1. Start the RAID configuration program. See 'Starting the RAID Configuration Program´ on page 17.
2. Select Advanced functions from the Main Menu.
3. Select Change write policy from the Advanced Functions menu. The cursor will be active in the Logical Drive list.
4. Select the logical drive whose write policy you want to change. The Advanced Functions screen appears.

Your information might be different from that shown in this screen.

The Logical Drive list shows you the logical drive ID, the size in megabytes of each logical drive, the RAID level you assigned to that logical drive, and the date you created it. The status of the logical drive is also shown. Good means that all is well with the drive; Critical means that you must replace the hard disk drive and rebuild the logical drive. (You will have received a message telling you what has happened to the drive.) Offline means that the logical drive is unrecoverable; the data in that drive is lost.
5. Locate the Wrt pol (Write Policy) field in the Logical Drive list. The write policy is shown as either WT (write-through, which is the default setting) or WB (write-back).
6. Use the Up Arrow (« ) key or the Down Arrow (» ) key to select the logical drive whose write policy you want to change.

If you change the write policy to write-back, wait at least 10 seconds after your last operation before you power-off the server. It takes that long for the system to move the data from the cache memory to the storage device. Failure to follow this practice can result in lost data.

7. Press Enter to change the write policy. Notice that WT changes to WB. You can press Enter to alternate between WT and WB.
8. When you have made your choice, press Esc to return to the Advanced Functions menu.
9. Select Exit. The Confirm pop-up window appears asking you to confirm your action.
10. To return the setting to its original state, select No. To save your changes, select Yes.
11. Back up the disk-array configuration information to diskette. Refer to 'Backing Up Your Disk-Array Configuration´ for more information.

Formatting Drives
You can perform a low-level format on drives with RDY (ready), OFL (offline), or UNF (unformatted) status.

The Format drive choice on the Advanced Functions menu provides a low-level format. If you install a new hard disk drive that requires a standard format, use the Format command provided by your operating system.

The Format program works like the low-level format program in the advanced diagnostics portion of the system programs. It is provided in the IBM RAID configuration program so that you can perform a low-level format on a drive controlled by the RAID adapter.

To perform a low-level format:
1. Start the RAID configuration program. See 'Starting the RAID Configuration Program´.
2. Select Advanced functions from the Main Menu.

A low-level format erases all data and programs from the drive.

3. Select Format drive. The low-level format program starts.
4. Follow the instructions on the screen.

You can perform a low-level format on more than one drive at a time.

Changing the RAID Parameters
You can change the RAID parameters using the advanced functions by selecting Change RAID parameters.

The default settings are: Stripe unit size -- 8K

The stripe unit size is the amount of data written on a given disk before writing on the next disk. To maximize the overall performance, this stripe unit should be chosen such that the stripe-unit size i close to the size of the system I/O request. The default is set to 8K data bytes.

Once the stripe unit is chosen and data is stored in the logical drives, the stripe unit cannot be changed without destroying data in the logical drives.

Rebuild priority -- Equal.

Rebuild priority can be set to equal, high, or low. When set to equal, the rebuild I/O request and system I/O request get equal priority in the execution order.

When set to high, the rebuild I/O request will get a higher priority than a system I/O request. In a heavily loaded system (with a high rate of system I/O requests), the high-priority rebuild can significantly reduce the disk rebuild time at the expense of degraded handling of I/O requests.

When the rebuild priority is set to low, the rebuild I/O requests can execute only if there is no pending system I/O requests. In a moderate to heavily loaded system, low rebuild priority will increase the disk rebuild time significantly and provide better system performance.

Rebuild priority can be changed without affecting data in the logical drives.

Parity placement -- RA.

Once a parity placement scheme is chosen and data stored, it cannot be changed without destroying data.

Parity placement defines how parity is placed in the disk array with respect to data. The following illustration shows both the Left Symmetric (LS) and Right Asymmetric (RA) parity placement in a four-drive disk array. Here AAA, BBB, and CCC are the data stripe units, and PP0 is the corresponding parity. Similarly DDD, EEE, and FFF are the data stripe units, and PP1 is the corresponding parity.

Right Asymmetric (RA) Left Symmetric (LS)
Disk Disk Disk Disk Disk Disk Disk Disk 
1    2    3    4    1    2    3    4

In some situations you may want to try LS parity placement to improve performance. The default parity placement is RA. Read ahead -- On. Normally the IBM SCSI-2 Fast/Wide PCI-Bus RAID Adapter transfers data from 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 local cache is equal to the system I/O request size, and no read-ahead to the end of the stripe unit is performed.

1. The Read-ahead setting can be changed without destroying data in a logical drive.
2. When the configuration is saved on a diskette, the RAID parameters are saved also.
Document ID:  DDSE-3UXNC8
Last Modified:  1999-06-09
Copyright 2000 IBM Corporation
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