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Changing the Write Policy (ServeRAID)



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. Under certain workloads, you can improve performance, by changing 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:
  1.  Start the ServeRAID configuration program (see 'Starting the ServeRAID Configuration Program')
  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.  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. OKY means that no problem conditions are associated with the drive, CRT 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.) OFL means  that the logical drive is irrecoverable, 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 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.

  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 the changes, select Yes.
  11.  Back up the disk-array configuration information to diskette.  Refer to 'Backing Up the Disk-Array'  for more information.


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