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INTRODUCTION
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runs as a background task. Auto Reassign Sector discontinues
when the operating system makes a request.
1.5.3 Consistency Check
A consistency check is a process that veries the integrity of
redundant data. For example, performing a consistency check
of a mirrored drive assures that the data on both drives of the
mirrored pair is exactly the same. To verify RAID 3, 5 or 6 redun-
dancy, a consistency check reads all associated data blocks, com-
putes parity, reads parity, and veries that the computed parity
matches the read parity.
Consistency checks are very important because they detect and
correct parity errors or bad disk blocks in the drive. A consistency
check forces every block on a volume to be read, and any bad
blocks are marked; those blocks are not used again. This is criti-
cal and important because a bad disk block can prevent a disk
rebuild from completing. We strongly recommend that you run
consistency checks on a regular basis—at least once per week.
Note that consistency checks degrade performance, so you should
run them when the system load can tolerate it.
1.6 Data Protection
1.6.1 BATTERY BACKUP
The SATA RAID controllers are armed with a Battery Backup Mod-
ule (BBM). While a Uninterruptible Power Supply (UPS) protects
most servers from power uctuations or failures, a BBM provides
an additional level of protection. In the event of a power failure, a
BBM supplies power to retain data in the RAID controller’s cache,
thereby permitting any potentially dirty data in the cache to be
ushed out to secondary storage when power is restored.
The batteries in the BBM are recharged continuously through a
trickle-charging process whenever the system power is on. The
batteries protect data in a failed server for up to three or four
days, depending on the size of the memory module. Under nor-
mal operating conditions, the batteries last for three years before
replacement is necessary.