Chapter 3. Understanding The Storwize V7000 Battery Operation For The Control Enclosure - IBM Storwize V7000 Maintenance Manual

Chapter 3. Understanding the Storwize V7000 battery
operation for the control enclosure
Storwize V7000 node canisters cache volume data and hold state information in
volatile memory.
If the power fails, the cache and state data is written to a local solid-state drive
(SSD) that is held within the canister. The batteries within the control enclosure
provide the power to write the cache and state data to a local drive.
Note: Storwize V7000 expansion canisters do not cache volume data or store state
information in volatile memory. They, therefore, do not require battery power. If ac
power to both power supplies in an expansion enclosure fails, the enclosure
powers off. When ac power is restored to at least one of the power supplies, the
controller restarts without operator intervention.
There are two power supply units in the control enclosure. Each one contains an
integrated battery. Both power supply units and batteries provide power to both
control canisters. Each battery has a sufficient charge to power both node canisters
for the duration of saving critical data to the local drive. In a fully redundant
system with two batteries and two canisters, there is enough charge in the batteries
to support saving critical data from both canisters to a local drive twice. In a
system with a failed battery, there is enough charge in the remaining battery to
support saving critical data from both canisters to a local drive once.
If the ac power to a control enclosure is lost, the canisters do not start saving
critical data to a local drive until approximately 10 seconds after the loss of ac
power is first detected. If the power is restored within this period, the system
continues to operate. This loss in power is called a brown out. As soon as the
saving of the critical data starts, the system stops handling I/O requests from the
host applications, and Metro Mirror and Global Mirror relationships go offline. The
system powers off when the saving of the critical data completes.
If both node canisters shut down without writing the cache and state data to the
local drive, the system is unable to restart without an extended service action. The
system configuration must be restored. If any cache write data is lost, volumes
must be restored from a backup. It is, therefore, important not to remove the
canisters or the power supply units from the control enclosures unless directed to
do so by the service procedures. Removing either of these components might
prevent the node canister from writing its cache and state data to the local drive.
When the ac power is restored to the control enclosure, the system restarts without
operator intervention. How quickly it restarts depends on whether there is a
history of previous power failures.
When the ac power is restored after a power outage that causes both canisters to
save their critical data, the system restarts only when the batteries have sufficient
charge to power both canisters for the duration of saving the critical data again. In
a fully redundant system with two batteries, this condition means that after one ac
power outage and a saving of critical data, the system can restart as soon as the
power is restored. If a second ac power outage occurs before the batteries have
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