IBM 86655RY Hardware Maintenance Manual - Page 219

combining Record 1 with the check sum data on Drive 3. At this point, the media sector error is discovered. • If the error is minor, the disk can re-create the missing information from its ECC data (as before) and potentially remap the bad sector. • If the error is too severe, the disk cannot recover the data. The rebuild process cannot complete successfully because it does not have a complete Record 1 to combine with the check sum data to rebuild the lost data on Drive 2. The Rebuild, therefore, skips that stripe and continues rebuilding the rest of the logical drive. Once the rebuild has completed, a "rebuild failed" message is displayed. The ServeRAID controller brings the rebuilt drive online and takes the array out of "critical" mode. To protect data integrity, it also blocks access to the damaged stripes of the array. Data files covered by these damaged stripes still report data errors and must be restored from a previous backup. The necessity for a full restore due to a "rebuild failed" message caused by one or two bad stripes is thereby prevented. Configuration procedures required to provide RAID-5 protection: Synchronization and initialization are required before installing an operating system or storing any customer data on a RAID-5 array for data integrity and to provide data protection. The RAID-5 arrays write data out to drives in stripe units. The size of the stripe unit can be configured to: • 8KB • 16KB • 32KB • 64KB Synchronization performs the following operations: • Reads all the data bits in each stripe unit • Calculates the parity for that data • Compares the calculated parity with the existing parity for all stripe units in the array • Updates the existing parity, if inconsistent Initialization writes zeroes out to the logical drive. Operating systems may not install properly if the first few sectors are not set to zero (approximately the first 32 sectors). After creating an array, the user must create a logical drive. Physical drives are the individual disk drives that are inside the machine. A logical drive is the drive that the array controller shows the operating system. A logical drive spans across multiple physical drives. All redundant RAID logical drives are automatically synchronized by the ServeRAID4 controller in the background when the logical drive is defined. No other user action is required. Providing additional protection for redundant RAID logical drives: Full tape backups and automatic data scrubbing provide protection against multiple drive failure and unprotected grown media defects. Full tape backups: Periodic full tape backups force all data files to be read and copied to tape. This procedure provides two advantages: • It provides an archive of all data in case it is needed to restore from a multiple drive failure or user error that could cause data loss/corruption. Installing and configuring ServeRAID controllers 209