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QUESTION 17 : Which process minimizes the exposure to a loss of uncommitted cached data when there is a power failure?

A. Cache vaulting

B. Cache mirroring

C. Low watermarking

D. Prefetching

Answer: A

Explanation: Cache Data Protection Cache is volatile memory, so a power failure or any kind of cache failure will cause loss of the data that is not yet committed to the disk. This risk of losing uncommitted data held in cache can be mitigated using cache mirroring and cache vaulting: 

Cache mirroring: Each write to cache is held in two different memory locations on two independent memory cards. If a cache failure occurs, the write data will still be safe in the mirrored location and can be committed to the disk. Reads are staged from the disk to the cache; therefore, if a cache failure occurs, the data can still be accessed from the disk. Because only writes are mirrored, this method results in better utilization of the available cache. In cache mirroring approaches, the problem of maintaining cache coherency is introduced. Cache coherency means that data in two different cache locations must be identical at all times. It is the responsibility of the array operating environment to ensure coherency.

Cache vaulting: The risk of data loss due to power failure can be addressed in various ways: powering the memory with a battery until the AC power is restored or using battery power to write the cache content to the disk. If an extended power failure occurs, using batteries is not a viable option. This is because in intelligent storage systems, large amounts of data might need to be committed to numerous disks, and batteries might not provide power for sufficient time to write each piece of data to its intended disk. Therefore, storage vendors use a set of physical disks to dump the contents of cache during power failure. This is called cache vaulting and the disks are called vault drives. When power is restored, data from these disks is written back to write cache and then written to the intended disks.

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