I. Explain the term prefetching. Why is it important?
II. Modern disks often have their own main memory caches, typically about one MB, and use this to do prefetching of pages. The rationale for this technique is the empirical observation that if a disk page is requested by some (not necessarily database!) application, 80 percent 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 disk's cache of several queries running concurrently, each scanning a different file.
3. Can the above problem be addressed by the DBMS buffer manager doing its own prefetching? Explain.
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 above problem? Given this technique, does it matter whether the DBMS buffer manager also does prefetching?