Dell PowerVault MD3820f - Manual (Page 15)

RAID Controller Module—Additional Features
Battery Backup Unit
Each RAID controller contains a two-cell Lithium ion nanopolymer battery backup unit (BBU). It provides
power to the RAID controller module in the event of a power outage. For information on removing and
installing the BBU, see
RAID Controller Module Backup Battery Unit .
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 re-enabled.
Storage Array Thermal Shutdown
The system automatically shuts down when system temperature exceeds the safe threshold. The battery
backup unit protects against data loss by providing power to offload cache to non-volatile memory in the
event of power loss. It is not necessary to shut down any MD1200 Series expansion enclosures attached
to the storage array when thermal shutdown occurs.
Temperature threshold values determine the temperature at which shutdown occurs. These thresholds
cannot be changed.
Table 1. Shutdown Threshold Type
Threshold Temperature Exceeding Event Description
Nominal failure threshold A critical event is set.
Maximum failure threshold Shutdown of the system power supplies occurs
within 3 minutes.
Shutdown threshold Shutdown of the system power supplies occurs
within 5 seconds.
System Password Reset
The storage array password can be reset if it is forgotten. To reset the password, push and hold down the
password reset switch for at least 5 seconds. The password is deleted.
The RAID controller module allows you to change the password.
NOTE: The reset switch can be accessed by using a small object such as the tip of a pen.
Cache Functions and Features
Cache Mirroring
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.
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