Cache Functions And Features - Dell PowerVault MD3000 Hardware Owner's Manual

Figure 1-6. RAID Controller Module
2
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 52.
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.

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.
16
About Your System
1
1 battery backup unit (BBU) tray 2 midplane connector (2)