font ean 13 para excel The Indexed-Sequential File in Software

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The Indexed-Sequential File
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Reducing the Access Costs of Indexes Two common improvements are made to indexes to increase their performance: 1 Keeping the root of the index tree in memory 2 Locating portions of the index so that seeks are minimized We will consider these two adaptations and then evaluate their joint e ect
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Root Blocks in Memory Since the root level of an index only occupies at most one
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block it is common to place it in memory, trading some memory for access speed Keeping the root block (level x) available, once it has been read, avoids repeated disk accesses for that level of an index The lower levels (levels x 1, , 1) of the index will still be obtained from disk The root block can be placed into memory when the le is opened With this scheme we reduce the total number of accesses to disk blocks required to fetch data records from the data le from x + 1 to x Since much of the time cost of fetching a block is due to the seek time required to reach a particular cylinder, access speed can be gained by avoiding seeks during access The primary index, having a parallel structure to the data le, can be partitioned according to hardware boundaries to yield such bene ts Speci cally, the top of the primary index (level x or levels x and x 1) can be used to partition the le into portions that each t one cylinder Two such levels are used only if the le is larger than a hardware device, say a disk unit Then the root level assigns the hardware device and the subsidiary level partitions the storage assigned to one device into cylinder portions On level 1 is the remainder of the index, its blocks allocated to the cylinders containing the data they reference Allocating index space within cylinders used for data slightly reduces the capacity for data and increases the bulk data transfer rate, but the access speed gained can be substantial To optimize this approach, the size of an index level is best determined by the number of blocks it should reference Only one low index level (1) needs to be allocated on a cylinder for its records; the number of entries is determined by the data capacity of the remainder of the cylinder More than one block may be needed for the entries to one cylinder, but since the blocks will be placed adjacent on a track, only additional bu er space and block transfer time (btt) is needed The next level up, level 2, matches the number of cylinders required Even if a le uses all of a disk unit, the number of entries will still be only a few hundred A level x = 3 only comes into play when les are greater than one disk unit The contents of the entries are also adapted to the hardware The optional third level uses as pointers references to the operating system directory of disk units The level two index contains only key attribute values and addresses of cylinder anchors If the level two entries match sequential cylinders the entries do not even need a pointer eld, since entry 1 simply corresponds to block 1, etc On the initial track of each cylinder there will be the cylinder index The entries on the cylinder index, level 1, do not need a cylinder address segment; they reference only local tracks, blocks, or records We have now a three- or two-level index, with possibly more fanout at lower levels than seen in a block-oriented index design No seek is incurred between
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Cylinder Indexes
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cylinder-index and data records For les residing on one disk a total of two seeks su ce for data access, one for the root block and one for the cylinder containing index and data The example shown in Example 3-7 evaluates this type of index for a large le and can be compared with Example 3-6
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Example 3-7
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