User`s guide

the impact of power failure is worsened by the possibility of write hole data loss
as well.
For this reason, the write-back cache differs from the read cache by incorporating
onboard rechargeable batteries. The batteries power the memory (to retain data)
when cache power is intentionally or accidentally interrupted. The battery circuit
automatically detects loss of power and switches from shelf backplane power to
battery power.
2.1.6.1 Battery Discharging
During a power failure, the write-back cache batteries discharge very slowly,
providing standby power to retain cached data. Under these circumstances, data
retention time for fully-charged batteries is guaranteed up to 100 hours. Data
may be lost if power is off for more than 100 hours (when starting with fully
charged batteries).
Studies performed by utility companies have noted that over 99 percent of all
power outages last less than 1 minute. However, once power fails for more than 3
minutes (with a 32-MB write-back cache) or 5 minutes (with a 16-MB write-back
cache), the controller will modify its operation upon restart as follows, in order to
maximize data preservation:
1. Non-RAID, disk-based units with write-back caching enabled will be accessed
in write-through (read cache) mode until the cache batteries are fully
recharged. Once the batteries are recharged, write-back caching will resume.
2. RAIDsets and mirrorsets, which under power failure situations may create
a write hole, cannot be served until the batteries are fully recharged if
the CACHE_POLICY=A qualifier is set. HSJ- and HSD30-based RAIDsets
will go ‘‘OFFLINE/INOPERATIVE’’ to the host until fully charged batteries
are available. An HSZ-based RAIDset’s LUN will appear to the host as
‘‘SCSI LUN NOT READY’’ until fully charged batteries are available. Cached
data is immediately flushed to the disk devices even while the batteries are
recharging.
CACHE_POLICY= A or B
For the HSOF V2.5 release, new cache policy qualifiers are available
for the SET THIS_CONTROLLER and SET OTHER_CONTROLLER
commands. These qualifiers are called CACHE_POLICY=A and
CACHE_POLICY=B.
If CACHE_POLICY=A is set, RAIDsets and mirrorsets are protected as
previously described in item 2.
If CACHE_POLICY=B is set, RAIDsets and mirrorsets are available in
write-though mode when write-back cache modules have low batteries. It
is important to note that if you choose to set CACHE_POLICY=B, data is
lost if the batteries fail (unless an uninterruptable power supply (UPS) is
used to keep the power turned on). Changing the policy on one controller
in a dual-redundant controller configuration automatically changes it on
the other controller. The change takes place immediately.
2–4 Controller Technical Description