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Basic File-System Organization Reorganization steps for an indexed-sequential le
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1 Read the le in the manner that would be used when doing serial processing, using both the sequential and the over ow areas 2 Leave out all records that are marked deleted 3 Write all new and old remaining records sequentially into the sequential areas of the new le 4 Create and insert into a memory bu er an index entry with every block placed into the sequential area 5 Write the index blocks out when they get full 6 Create and insert a higher level index entry for every index block written 7 When all data blocks have been written the remaining index bu ers are written out 8 Free the areas used by the old index The frequency of reorganization is dependent on the insertion activity within the le In practice, one nds time intervals ranging from a day to a year between reorganization runs Since a reorganization run can take a long time, the reorganization is generally performed before the le is actually full to avoid unpleasant surprises at busy times It may simply be scheduled at periodic intervals or be done at a convenient instance after the entries in the over ow area exceed a preset limit In Chap 14-5 an algorithm for determining reorganization intervals will be presented 3-3-2 Use of Indexed-Sequential Files Indexed-sequential les of the basic type discussed above are in common use in modern commercial processing They are used especially where there is a need to keep les up to date within time frames that are less than the processing intervals which are possible with cyclical reorganization of sequential les Since individual records can be inserted and retrieved through the index, so that a limited number of block accesses are required, this type of le is suitable for on-line or terminaloriented access On-line use is not feasible with the pile and sequential le types unless the les are quite small At the same time sequential access is relatively simple and e cient Without over ows, after a reorganization, sequential access is practically as fast as for the sequential le An indexed-sequential le can, for instance, be used to produce an inventory listing on a daily basis and be reorganized on a weekly basis in concert with a process which issues notices to reorder goods for which the stock is low Indexed-sequential les are also in common use to handle inquiries, with the restriction that the query must specify the key attribute Typical of these are billing inquiries based on account numbers Sometimes copies of the same data are found sequenced according to di erent keys in separate indexed-sequential les to overcome this restriction Updating cost and space requirements are multiplied in that case The e ects of the speci c design implemented for an indexed-sequential le are frequently not understood by the users, so that many applications which use
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Sec 3-3
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The Indexed-Sequential File
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indexed-sequential les take longer to process data than seems warranted In situations where les receive updates in clusters, the generated chains can be costly to follow Often, clustered updates are actually additions to the end of a le By treating these separately, or by preallocating space and index values in the indexedsequential le during the prior reorganization, the liabilities of appended records can be reduced Within one indexed-sequential le method the options are often limited, but a number of alternative indexed-sequential le implementation are available from computer manufacturers and independent software producers The restriction that only one attribute key determines the major order of the le, so that all other attribute values are not suitable as search arguments, is common to all sequential les Indexing with multiple keys is presented in Sec 3-4 There a di erent scheme of index management is used and the sequentiality of the le is abandoned
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3-3-3 Performance of Indexed-Sequential Files Performance evaluation of indexed-sequential les is more complex than evaluation of the two preceding le organization methods because of the many options possible in the detailed design We will base our evaluation on a simple form similar to the most common commercial designs The index is on the same key attribute used to sequence the data le itself The rst-level index is anchored to blocks of data, and a secondlevel index has one entry per rst-level index block Push-through is used when records are inserted into a block Records in an over ow area of size o are linked in key order to provide good serial access Records to be deleted are not actually removed, but only marked invalid with a tombstone An additional eld in each record of the main le is used for the tombstone A eld for a pointer for chaining is maintained within the over ow area For both functions the same eld position, of size P , is allocated in the records We permit records of variable length in our analysis, although only few systems support such records After a reorganization the main data areas and index areas are full and the over ow areas are empty No change of the index takes place between reorganizations, simplifying the insertion of records All areas are blocked to an equal size B The total le may occupy multiple cylinders In the sequential part of the le, a record requires space for a data values of size V and for a possible tombstone of size P R = aV + P net 3-28
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