Recovering Data From Battery-Backed Cache; Selection Criteria For Battery-Backed Cache; Types Of Batteries - HP 273914-B21 - Smart Array 6404/256 RAID Controller Technology Brief

Recovering data from battery-backed cache

If an unexpected server shutdown occurs while data is held in BBWC, Smart Array controllers
automatically signal the memory chips to enter a self-refresh state and the controller initiates battery
power, or system auxiliary power if present. An amber LED, available either on the cache module or
battery pack, begins flashing to indicate that data is trapped in the cache. Smart Array controllers
automatically write this data to disk when power is restored to the system. If power is not restored
within the specified backup duration, the batteries may become drained of power. If that happens,
posted-write data in the cache will be lost. Once system power is restored, the batteries will
automatically recharge if needed. Battery recharge takes between 30 minutes and 2 hours,
depending on the remaining capacity level.
In the event of a server failure, the Smart Array controller and all of the drives can be moved to
another server to allow writing the data in the write cache to the drives.
In the event of a controller failure, the cache module containing posted-write data can be moved to a
new Smart Array controller. However, to preserve the cached data, the new Smart Array controller
must be attached to the original drives for which the posted-write data is intended.
Administrators should be aware of a special concern when using an embedded RAID controller with
battery-backed cache. If the server board fails, the replacement board must be the same model server
board so that the controller type and drive bays are the same. The cache module, battery pack, and
drives must be moved to the replacement system to extract the data from the battery-backed cache.

Selection criteria for battery-backed cache

HP Smart Array battery cells, battery enclosures, and contacts are custom designed to preserve the
integrity of business-critical information beyond the minimum specified backup duration. HP Smart
Array battery cells were selected to achieve the specified three-year backup life in typical server
environments.
A dedicated battery microcontroller continuously monitors the HP Smart Array battery pack for signs
of damage, including an open battery terminal, partial battery short, charge timeouts, and over
discharge conditions. Battery status information is indicated with an LED, power-on self-test (POST)
messages, event messages to the host, ACU information pages, ADU, and within HP SIM.
The battery microcontroller automatically disables the battery-backed cache features any time it
detects battery damage or the charge level falls below the required limits to achieve the specified
backup duration. The battery microcontroller automatically restores battery-backed cache features
when the microcontroller detects a replacement battery or when battery recharging is complete. High-
end HP RAID controller designs contain two batteries to protect against a single battery cell failure
while data is held in cache.
For detailed technical information on all HP cache options and controller compatibility, go to
www.hp.com/products/smartarray.

Types of batteries

HP Smart Array controllers use rechargeable Nickel Metal Hydride (NiMH) button cell batteries
specifically designed for longer life at the temperatures found inside rack-mounted servers. Typical
capacity reduction for the HP Smart Array battery pack is 5 to 10 percent over a 3-year period,
depending on server temperature and number of discharge cycles.
NiMH cells are also environmentally friendly, since they do not contain harmful lead, mercury, or
cadmium material. Additionally, NiMH chemistry does not suffer capacity memory effects that can
lower battery capacity. For example, memory capacity of Nickel Cadmium (NiCD) batteries is
reduced when the batteries are exposed to short discharge cycles. Lithium Ion (Li-Ion) batteries are
typically smaller than NiMH batteries, but their capacity is permanently reduced at high temperatures
and they are usually limited to 100 full discharge cycles.
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