VSAM Files
The physical organization of VSAM data sets differs considerably from the organizations used by other access methods is as follows.
VSAM data sets consist of control intervals (CI) and control areas (CA). The size of the CI and CA is normally determined by the access method, and the way in which they are used is not visible to the user. There will be a fixed number of control intervals in each control area.
A control interval normally contains multiple records. The records are stored within the control interval starting from the low address upwards. Control information is stored at the other end of the control interval, starting from the high address and moving downwards. The space between the records and the control information is free space. The control information comprises two types of entry: a control interval descriptor field (CIDF) which is always present, and record descriptor fields (RDF) which are present when there are records within the control interval and describe the length of the associated record. Free space within a CI is always contiguous.
When records are inserted into a control interval, they are placed in the correct order relative to other records. This may require that records have to be moved up the control interval. Conversely, when a record is deleted, later records are moved down so that the free space remains contiguous. If there is not enough free space in a control interval for a record to be inserted, the control interval is split. Roughly half the records are stored in the original control interval while the remaining records are moved into a new control interval. The new control interval is taken from a pool of free control intervals within the same control area as the original control interval. If there is no remaining free control interval within that control area, the control area itself is split and the control intervals are distributed equally between the old and the new control areas.
You can use three types of record-orientated file organization with VSAM (the contents of linear data sets have no record structure):
Read more about this topic: Virtual Storage Access Method
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