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RAID-1 and RAID-5 provide excellent protection in single drive failures. However, what can be
done to protect against multiple drive failures and undetected grown defects ? Since the real
source of failed rebuilds described above is undetected sector media errors, a good approach to
avoid problems is to periodically force the detection of any sector media errors through Data
Scrubbing both RAID-5 and RAID-1 logical drives. This can be done in several ways.
Full Tape Backups
Periodic full tape backups will force all data files to be read and copied to tape. This procedure
provides two advantages. First, it obviously 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. Secondly,
by reading every file on the disk, this process forces the disk and the array to discover any sector
media errors and correct them, greatly decreasing the likelihood of the rebuild failures described
in the previous section. If weekly tape backups are performed, Data Scrubbing is still
recommended to detect and correct sector media errors in non-data areas of the drives.
If a sector media error has not been discovered and an array rebuild fails as described previously,
a full tape backup can then be performed to minimize data loss. The tape backup will save all
files except for the one containing the damaged sector. The drives can then be restored from the
configuration disk and synchronized - forcing the remapping of bad sectors and the data can be
restored. The files that were contained on the damaged sector would then be restored from a
prior backup. Refer to 'Ensuring High Availability of Your RAID Subsystem with IBM SCSI-2
Fast/Wide PCI-Bus RAID Adapter, IBM Fast/Wide Streaming RAID Adapter' if you do not
have a backup configuration diskette.
Periodic Data Scrubbing
Another way to force discovery and correction of sector media errors is Data Scrubbing. With
the IBM ServeRAlDIl Adapter, Data Scrubbing is performed by the adapter Firmware as a
background process. For all other IBM RAID Adapters, Data Scrubbing can be performed by
synchronizing the logical drives in each array. Array synchronization reads the data and parity
blocks from all the disks and checks that the parity correctly matches the data block information.
If the logical drive was synchronized upon configuration, the parity information will match the
data. Therefore, the synchronization procedure is actually just being used as a Data Scrubbing
tool. The advantage to this procedure, when used in addition to tape backups, is Data Scrubbing
will also examine unallocated sectors of the drive in addition to those that have been allocated
(data areas are protected by a tape backup). If any sector media errors are found during this
process, they are corrected at the disk level using disk FCC information if possible, or at the array
level using RAID parity information if necessary. IBM recommends Data Scrubbing be
performed weekly to ensure a high level of data protection.
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