Product manual

Managing Critical Data
77 A1 69UP Rev00 5-5
The backup battery subsystem allows CDA 7 to remain online to the host system
for three minutes in the event of an AC power loss. This three minute window
allows CDA 7 to support frequent power outages because the battery is able to
fully recharge during Initial Microcode Load (IML) time. CDA 7 continually
recharges the battery subsystem whenever it is under AC power.
The battery backup also prevents disk device failures due to the sharp power drops
that occur during unexpected power interrupts. If power is restored before the
battery timer expires, CDA 7 becomes available again without IML.
5.2.3.1 CDA 7 Power Failure on GCOS 7 Channels Connected to GCOS 7
When a power failure occurs, power switches immediately to the backup battery
and CDA 7 continues to operate normally. When the battery timer window elapses,
CDA 7 presents a busy status to prevent the host system from writing or reading
any data at the unit. CDA 7 destages any Fast Write data still in cache to disk,
spins down the disk devices and retracts the heads, and powers down, turning off
the battery at that time. CDA 7 will not respond to SCSI selections after it powers
down.
5.2.4 Dual-Initiator Feature
CDA 7 has a dual-initiator feature that ensures continuous availability of data in
the unlikely event of a CDA 7 disk management hardware failure. It does not
provide data availability in the event of a disk device failure. This feature works
by having two disk directors “shadow” the function of the other. That is, this
feature gives each disk director the capability of servicing any or all of the disk
devices of the disk director with which it is paired when one disk director is
completely servicing its own and the shadowed devices.
With dual-initiator, the disk directors work in pairs. See the figures on the
following pages. Under normal conditions, each disk director services its disk
devices. If the sophisticated fencing mechanisms of CDA 7 detect a disk
management hardware failure, CDA 7 notifies the host and the Bull Competence
Center of the failure, and reads from or writes to the disk devices that its disk
director pair was unable to communicate with without interruption. When the
source of the failure is corrected, CDA 7 returns the I/O servicing of the two disk
directors to their normal state.