RAID HDDs and Synchronization
Before installing software or data for the first time on an IBM PC Server RAID system, the following must be performed:
RAID synchronization is recommended as a preventive maintenace procedure to reduce the risk of an array Rebuild-failure. IBM recommends that synchronization be run weekly to provide a high level of protection. The level of protection increases as more frequent synchronizations are performed.
To reduce the frequency of synchronizations to once or twice a month and still maintain a high level of protection, schedule synchronizations along with other preventative maintenance procedures like regular tape backups.
Over time a hard disk may accumulate grown defects. This is normal. Defects are corrected on accessed files by RAID and by the disk subsystems. If a grown defect is encountered when a file is accessed, both the data and parity information are corrected using the information striped across all drives. However, if a grown defect appears on an area that is not accessed (the area is free space, or because the file is accessed from cache), then synchronization is required to reconstruct the data. If all drives are functional, the defect has a grown defect, and another drive has failed completely, then there is not enough information to reconstruct the data and the Rebuild will fail.
Synchronization can help to identify potential drive failures because it scrubs every block on a logical drive. Synchronization forces all data in the logical drive to be read regardless of whether the data is part of a valid file or just free space. The RAID adapter recomputes the parity based on the data it reads and re-writes the parity for RAID-5 or just rewrites the mirror copy of data for RAID-1. The RAID adapter reconstructs any data that it cannot read due to grown defects and re-writes/reassigns the data so that it can be read without error later. Then, if a drive fails, the Rebuild will complete.
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