occurs, the cache operates in write-through mode and writes directly to the
drives to prevent slow data throughput via the cache. When the battery
charge is 50% or more, the cache write through mode is turned off and the
system operates normally.
If a power failure occurs and continues for up to 20 milliseconds, the storage
system continues normal operation. If the power failure exceeds 20
milliseconds, the storage system uses power from the batteries to keep the
cache active while the system copies the storage system configuration and
cached data to the cache flash memory in the cache backup modules. The
process continues up to 32 minutes. The cache flash drives do not require
power to retain the data.
The following illustration shows the timing if a power failure occurs.
Item
Description
1 Power failure occurs.
2 The storage system continues to operate for 20
milliseconds and detects the power failure.
3 The cache memory data and storage system
configuration are backed up to the cache flash
memory in the cache backup assemblies. If
power is restored during the backup, the backup
stops unless the backup battery capacities are
used less than 50%. In case, the system
operates in write-through mode until the
batteries are charged enough for a full backup.
4 Data is stored in the cache flash memory until
power is restored, then it is written to the
drives.
Cache backup batteries
If a power failure occurs, nickel metal-hydride batteries keep the cache
memory active while the storage system configuration and data in the cache
is copied to the flash memory in the cache backup modules. The batteries are
located in the cache backup modules. They are fully charged at the
distribution center where the storage system is assembled and tested. During
Power on/off procedures 113
Hitachi Virtual Storage Platform G1000, G1500, and Virtual Storage Platform F1500 Hardware Guide
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