An Overview of Drive Replacement

An Overview of Drive Replacement

For the IBM ServeRAID Adapter, you replace drives via the RAID configuration utility. To begin, select the Rebuild Drive option under Rebuild/Device Management option on the RAID Configuration Main Menu. The Rebuild Drive command requires that all drives in the array (except the drive being rebuilt) are online. When you select Rebuild Drive, the RAID adapter sends a start unit command to the drive being rebuilt. Once the drive starts successfully, the drive state changes from defunct (DDD) to rebuild (RBL). The logical drive remains in a critical (CRT) state until the rebuild is completed. Once the rebuild completes successfully, the logical drive changes to the okay (OKY) state.

If the logical drives are in an offline (OFL) state, meaning that multiple drives in an array are defunct, then you must use the Set Device State to ONL on all drives except the 'inconsistent' drive before rebuilding. If you mistakenly use this command on the 'inconsistent' drive, the rebuild process corrupts data.
NOTE: Please review the following section, 'Software Replace vs. Physical Replace', as well as the recovery procedures sections later in this document to understand which drive in a scenario is the 'inconsistent' drive. Knowing how to determine the 'inconsistent' drive is extremely important. This knowledge ensures you can troubleshoot correctly without corrupting data.

After you select the Rebuild Drive option, the next prompt asks you to indicate whether you want to rebuild the drive in the same location or a new location. Select Same Location if you physically replaced the old hard disk drive with a new one in the same bay. Select New Location to assign a hard disk drive in a new location. After the drive location has been selected, the adapter sends a start unit command to the drive and begins the rebuilding process on the critical logical drives in the array. The rebuild completes quickly under the following circumstances:

if the defunct (DDD) drive was not defective
if the drive is in the same bay location
if no write operations or configuration changes occurred that require rebuilding data in that drive

During the rebuilding process, the IBM ServeRAID Adapter reports the drive state as RBL. Once the rebuild completes successfully, the drive state changes from RBL to ONL and the logical drive state changes from CRT to OKY.

A DDD drive can occur in cases where the adapter is unable to determine the root cause failure. If the drive is not actually defective, you can use the rebuild option to resolve a DDD state without the need to replace the drive physically. This type of drive replacement where the drive is not physically replaced is called a Software Replace.

Software Replace vs. Physical Replace

When the RAID Adapter communicates with the hard file and receives an unexpected response, the adapter will mark the drive defunct in order to avoid any potential data loss. For example, this could occur in the event of a power loss to any of the components in the SCSI RAID subsystem. In this case, the RAID adapter will error on the side of safety and will no longer write to that drive although the drive may not be defective in any way.

These circumstances warrant either a software replace or a physical replace, as discussed in the following bullets:

Using a soliware replace is recommended to try to recover data when multiple DDD drives occur. In this situation, you may lose data on drives that are not actually defective if you run a normal rebuild process.
IF YOU USE THE WRONG ORDER WHEN YOU ATTEMPT A SOFTWARE REPLACE, DATA CORRUPTION RESULTS.
A software replace should also be performed for a single DDD drive when a hot spare (HSP) is not present in the system and it is the first time the drive has been marked DDD. In this situation, the software replace requires a rebuild of the drive. During the rebuild, all sectors of the drive are rebuilt. Therefore, the drive is tested very well. If a rebuild of the drive completes successfully, the drive does not need to be physically replaced.
If a DDD drive has been replaced by an HSP, you should physically replace the DHS drive. Under these circumstances, a software replace will only send a start unit command to the drive. If the unit starts successfully, then the drive is seen as good by the RAID adapter. Just restarting a drive does not sufficiently test the drive. Therefore, the drive should be physically replaced to ensure a good HSP drive is present in the system.

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