By C.K. Wong

A significant technological pattern for giant database structures has been the advent of ever-larger mass garage platforms. this enables computing facilities and enterprise info processing installations to keep up on-line their software libraries, much less often used information records, transaction logs and backup copies less than unified approach regulate. Tapes, disks and drums are classical examples of mass garage media. The newer IBM 3851 Mass garage Facility, a part of the IBM 3850 Mass garage procedure, represents a brand new course in mass garage improvement, specifically, it really is two-dimensional. With the adulthood of magnetic bubble know-how, extra refined, gigantic, multi-trillion-bit garage platforms are usually not a ways sooner or later. whereas huge in ability, mass garage platforms have usually really lengthy entry occasions. considering checklist entry chances usually are not uniform, quite a few algorithms were devised to place the files to diminish the typical entry time. the 1st chapters of this ebook are dedicated commonly to such algorithmic stories in linear and two-dimensional mass garage structures. within the 3rd bankruptcy, we view the bubble reminiscence as greater than a garage medium. in reality, we speak about assorted constructions the place regimen operations, akin to facts rearrangement, sorting, looking out, etc., might be performed within the reminiscence itself, liberating the CPU for extra complex initiatives. the issues mentioned during this publication are combinatorial in nature.

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**Extra info for Algorithmic Studies in Mass Storage Systems**

**Sample text**

Fortunately, the heuristic placement was able to pass the stronger test of optimality, in which we compared with the optimal static placement strategy when the frequencies associated with all users are known a priori. It may be that stronger optimality statements could be made if one considers the weaker test, in which we restrict ourselves to the class of algorithms that must take placement decisions in a step-by-step fashion as arrivals occur. 1 do not depend on the assumption of a frequency distribution for the ft's.

Where f(e,n) is a function of just e and n. Let + (1 x 'IT' - e) + 2f(e,n), be the arrangement where the record with access probability e is placed in the middle. We have D( 'IT') = x + (1 - e) + [x(1 - e)2]/(2e) g(e,n) is again a function of e and n only. + 2g(e,n), where By choosing x large enough, the ratio D('IT')/D('IT") can be made to approach l/(2e) e/2 arbitrarily closely. + Since e = min {Pj}, we have the desired 1 ~j~n result. 2 shows that an arbitrary arrangement can perform quite poorly in comparison with an optimal arrangement.

In fact, we shall prove that this problem is NP-Complete [6,12]. To this end, we need a more formal description of the problem. An allocation or arrangement of the records is a permutation of the indices 1 to n and is denoted by 'IT = ('lTI, ... ,'lT n ), where 'lT j is the index of the record assigned to the i-th position from the left. 1) where d(i,j) is the distance traveled in accessing the record at position j, given that the record at position i was last accessed. Consider the problem of allocating a set of records to a magnetic tape.