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Write-back is a cache-writing strategy in which the controller receives
the data to be written to disk, stores it in the memory buffer, and immediately
sends the host OS a signal that the write operation is complete, without
waiting until the data is actually written to the disk drive. Within a
short time, the controller, when it is not busy, writes the data to the
disk drive. Write-back caching improves the performance of write operations and the
throughput of the controller card. However, because there is a danger of data loss in the case of a power failure,
storage systems with write-back caching should be equipped with a UPS or
battery backup cache. A UPS provides power long enough to enable any data
in the cache memory to be written to the disk drive. With battery backup
cache, the battery provides power to retain the memory data for up to 48
hours. |