Dell PowerVault MD3000 - Manual (Page 17)

About Your System
17
Battery Backup Unit
Each RAID controller contains a three-cell lithium-ion battery backup unit (BBU) that powers the
controller’s cache memory and preserves the cache contents in the event of a power outage of up to
72 hours. The RAID controller firmware performs a test of the BBU at startup and will illuminate the
battery fault LED if the battery is not operating within specified ranges, or if the battery is missing. For a
description of the battery fault LED, see Table 1-5. For information on removing and installing the BBU,
see "Removing and Installing a RAID Controller Module Backup Battery Unit" on page 60.
NOTE:
For virtual disks, the RAID controller firmware changes the data cache setting based on the state of the
battery. If the battery is missing or does not have sufficient charge, the controller flushes the cache and sets the
write cache attribute to Write Through for all virtual disks. When the battery is replaced, Write Back is reenabled.
The RAID controller module logs the age of the battery and issues a replacement reminder message
approximately six weeks before expiration. After replacing the battery, you must use MD Storage
Manager to reset the battery age.
RAID Enclosure Thermal Shutdown
Enclosure management provides a feature that automatically shuts down the enclosure when the
temperature within the storage enclosure exceeds a safe threshold. Thermal shutdown protects the data
on the physical disks from corruption in the event of cooling system failure. It is not necessary to shut
down any expansion enclosures attached to the storage enclosure.
The enclosure temperature probe’s Nominal Failure Threshold and Maximum Failure Threshold
determine the temperature at which shutdown occurs. These thresholds are default settings that cannot
be changed. If the temperature sensors on the backplane or RAID controller module detect a
temperature exceeding the Nominal Failure Threshold, a critical event is set; if the Maximum Failure
Threshold is hit, shutdown of the enclosure power supplies occurs within 5 seconds.
Cache Functions and Features
Cache Mirroring
The cache mirroring function copies accepted host-write data from the primary controller to the partner
controller. This action ensures that host-write data is safely mirrored to the partner controller before
successful completion status is returned to the host. If a controller fails, the surviving controller safely
retains all mirrored data. Cache mirroring is enabled by default.
Write-Back Cache
Write-back cache is a caching strategy whereby write operations result in a completion signal being sent
to the host operating system as soon as the cache receives the data to be written. The target physical disk
will receive the data at a more appropriate time in order to increase controller performance. In dual-
active controller configurations with write-back caching enabled, the write data is always copied to the
cache of the second controller before completion status is issued to the host initiator. Write Back is
enabled by default.