Backup School Quiz 1, Answer # 11

Here is the answer to Backup School, quiz 1, question # 11.

Question #11

When writing data to the cache:

a. Only the affected blocks are changed
b. All of the blocks are changed
c. None of the blocks are changed
d. Empty blocks are changed

Were you correct? The correct answer is:

a. Only the affected blocks are changed

Learn more:

Operating system and storage vendors have implemented caching in an effort to maximize system throughput. In general, smaller I/O requests (read or write) have a noticeable impact on system throughput. The amount of instructions required to be executed to transfer -- say 512 bytes of data -- is approximately the same as, 64 kb of data. Also, each request involves a number of context switches. This means that the overhead of transferring data in smaller request sizes can be appreciably more. To alleviate this, operating system and disk manufacturers have implemented caching. On hardware devices, this involves some amount of faster access memory, usually RAM, that buffers requests and in some cases performs other optimizations. One of the ways caching enhances system throughput is by coalescing small I/O requests into fewer bigger I/O requests. Another way caching enhances system throughput is by allowing parallel operations. One I/O request may be accomplished between spinning media and cache while a second I/O request is simultaneously completed between an application data buffer and the cache.

While caching can enhance system throughput, some forms of write caching can increase the chances for data corruption. Data within a cache may be lost for a number of reasons including power failure, a system or bus reset, etc. In general, essential data such as database log files or file system meta-data should not be cached.


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This was first published in April 2004

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