The disk controller on each storage server periodically performs a discharge and charge of the controller battery. During the operation, the write cache policy changes from write-back caching to write-through caching. Write-through cache mode is slower than write-back cache mode. However, write-back cache mode has a risk of data loss if the storage server loses power or fails. For storage server releases earlier than release 220.127.116.11, the operation occurs every month. For Oracle Exadata Storage Server Software release 18.104.22.168 and later, the operation occurs every three months, for example, at 01:00 on the 17th day of January, April, July, and October.
CellCLI> ALTER CELL bbuLearnCycleTime="2011-01-22T02:00:00-08:00"
The time reverts to the default learn cycle time after the cycle completes.
CellCLI> LIST CELL ATTRIBUTES bbuLearnCycleTime
The storage server generates an informational alert about the status of the caching mode for logical drives on the cell, for example:
HDD disk controller battery on disk controller at adapter 0 is going into a learn cycle. This is a normal maintenance activity that occurs quarterly and runs for approximately 1 to 12 hours. The disk controller cache might go into WriteThrough caching mode during the learn cycle. Disk write throughput might be temporarily lower during this time. The message is informational only, no action is required.
# /opt/MegaRAID/MegaCli/MegaCli64 -AdpBbuCmd -GetBbuStatus -a0
BBU status for Adapter: 0 BatteryType: iBBU08 Voltage: 3721 mV Current: 541 mA Temperature: 43 C BBU Firmware Status: Charging Status : Charging Voltage : OK Temperature : OK Learn Cycle Requested : No Learn Cycle Active : No Learn Cycle Status : OK Learn Cycle Timeout : No I2c Errors Detected : No Battery Pack Missing : No Battery Replacement required : No Remaining Capacity Low : Yes Periodic Learn Required : No Transparent Learn : No Battery state: GasGuageStatus: Fully Discharged : No Fully Charged : No Discharging : No Initialized : No Remaining Time Alarm : Yes Remaining Capacity Alarm: No Discharge Terminated : No Over Temperature : No Charging Terminated : No Over Charged : No Relative State of Charge: 7 % Charger System State: 1 Charger System Ctrl: 0 Charging current: 541 mA Absolute State of Charge: 0% Max Error: 0 % Exit Code: 0x00