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Servers - Synchronization to prevent RAID rebuild failure
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Abstract
RAID array synchronization - Prevent RAID rebuild failure

Summary
When a hard drive fails and is replaced in a RAID-1 or RAID-5 array, a Rebuild failure may occur if there is a sector on one of the remaining working drives that cannot be read.

RAID-1 and RAID-5 logical drives must be synchronized immediately after they are created to assure that the mirror copies of the data (RAID-1) or parity data stripes (RAID-5) are correct and match the associated blocks that they are intended to protect.

RAID synchronization is recommended as a preventative maintenance 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 is reallocated and the data is recovered. However, if a drive 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.

In addition, the RAID Adapter Firmware level should be checked and updated if necessary. The PCI F/W RAID Adapter requires Firmware 2.43 or later. The MicroChannel F/W Streaming RAID Adapter/A requires Firmware 2.21 or later.

SAS Keywords :
PSY2, PSY2ADPT, D/T8640, D/T8642, 320, 06H5078, 06H3059, 92F0335, 06H9334, DDD, DEFUNCT, 520, 720, SERVER, 500
Document ID:  RMIE-3A2CXD
Last Modified:  1999-09-11
Copyright 2000 IBM Corporation
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