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Unformatted text preview: Storing Data: Disks and Files 85 Exercise 9.15 Modern disks often have their own main memory caches, typically about 1 MB, and use this to prefetch pages. The rationale for this technique is the em- pirical observation that, if a disk page is requested by some (not necessarily database!) application, 80% of the time the next page is requested as well. So the disk gambles by reading ahead. 1. Give a nontechnical reason that a DBMS may not want to rely on prefetching controlled by the disk. 2. Explain the impact on the disks cache of several queries running concurrently, each scanning a different file. 3. Is this problem addressed by the DBMS buffer manager prefetching pages? Ex- plain. 4. Modern disks support segmented caches , with about four to six segments, each of which is used to cache pages from a different file. Does this technique help, with respect to the preceding problem? Given this technique, does it matter whether the DBMS buffer manager also does prefetching? Answer 9.15 1. The pre-fetching done at the disk level varies widely across different drives and manufacturers, and pre-fetching is suciently important to a DBMS that one would like it to be independent of specific hardware support. 2. If there are many queries running concurrently, the request of a page from different queries can be interleaved. In the worst case, it cause the cache miss on every page request, even with disk pre-fetching. 3. If we have pre-fetching offered by DBMS buffer manager, the buffer manager can predict the reference pattern more accurately. In particular, a certain number of buffer frames can be allocated per active scan for pre-fetching purposes, and interleaved requests would not compete for the same frames.interleaved requests would not compete for the same frames....
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- Fall '12