Changing the Write Policy

^{IBM-AUSTRIA - PC-HW-Support 30 Aug 1999}

Changing the Write Policy

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.

If you change the write policy to write-back, wait at least 10 seconds after the 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.

To change the write policy:

Start the RAID configuration program by inserting the IBM SCSI-2 Fast/Wide PCI-Bus Adapter Configuration Option Diskette into the primary diskette drive and turning on the system. If the system already is turned on, press Ctrl+Alt+Del.
Select Start RAID Configuration Program from the PC DOS start-up menu and press Enter.
Select Advanced functions from the Main Menu.
Select Change write policy from the Advanced Functions menu. The cursor will be active in the Logical Drive list.
Select the logical drive whose write policy you want to change. A screen similar to the following appears:

NOTE: The information might be different from that shown in this screen.
The Logical Drive list contains 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.
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).
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 the 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.
Press Enter to change the write policy.
Notice that WT changes to WB. You can press Enter to alternate between WT and WB.
When you have made your choice, press Esc to return to the Advanced Functions menu.
Select Exit. The Confirm pop-up window appears asking you to confirm your action.
To return the setting to its original state, select No. To save the changes, select Yes.
Back up the disk-array configuration information to diskette. Refer to 'Backing Up the Disk-Array' for more information.

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