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Directory A directory to the allocated portions is kept as part of an entry for every vsam le in a vsam le catalog The directory is limited to 256 entries A possible allocation is shown in Fig 4-14 The size of the allocation to the initial and to any further portions is determined by the expected maximum le size, as estimated from parameters given at the original le de nition time These parameters also are kept in the le catalog entry The entire catalog contains many le descriptions and is itself a vsam le
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The allocation within the portions is also done using the B-tree algorithm The blocking and insertion of records into trains will be presented below Blocking vsam uses the train concept presented in Sec 2-3-1 to achieve blocking
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Each train has a xed number of blocks, also determined at the time of initial allocation, and is treated as one logical unit Within a portion there will be space for a number of trains Some of these may be in use; others may be free Trains in use contain a variable number of records The records may be of variable length and may span blocks within a train A partially lled train is shown in Fig 4-15 The actual data records are packed into the forward area of the train; the three-byte markers giving the length of these records are stored in reverse order in the rear of the train Adjoining records of equal size are controlled by a single, double-sized marker A control eld for the entire train gives the amounts of space used by the markers and left free Record Insertion The free space in a train can be used to insert records and position information without requiring index manipulation The new record is merged into its proper position Some shifting within the train will nearly always take place to achieve this The markers may have to be shifted too Since they contain only the length, old markers retain their values If there is insu cient space in the train, a free train will be requisitioned The B-tree algorithm is carried out in three steps, performed in a sequence which reduces vulnerability of the data in case of system malfunction 1 The records comprising the rst half of the full train are copied into the new train, and the new train is written out to disk 2 The index is updated to include the new train at the appropriate point; the last record provides the key value for the index entry 3 The old train is adjusted by shifting the latter half of the records to the front, and this train is rewritten During this sequence, the record which was to be added is put into the correct position, and the length markers are set for the old and the new train We already have discussed the measures taken when there is no free train in the portion, so that a new portion has to be allocated Both over ow handling algorithms are B-tree based; they di er mainly in terms of the size of the units that are being manipulated Loading Density To allow updating of the les without immediately forcing the splitting of trains and portions, the initial le density is speci ed at < 1 Here the B-tree algorithm is used for an indexed sequential le, trading space in order to avoid deterioration of performance due to insertions Reorganization is not normally needed A le which undergoes much updating will eventually achieve an equilibrium similar to the equilibrium conditions described in Eq 3-47 We used the result that space utilization will approach 69% so that we expect the trains eventually to be 69% occupied In addition, there will be some storage loss from unused trains in portions The directory to the trains in one portion provides for the allocation of initial free trains Trains which are freed are made available for reuse only within the same portion, so
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