ISAM

ISAM, promoted by IBM and Ingres DBMS, also uses the linear indexing philosophy of primary key sorting. In ISAM, files on disk are divided into cylinders on disk. The cylinders are tracks readable from a particular placement of the head on a multiple platter of disk drive. ISAM addresses the limitation of linear indexing of insertion or deletion to some extent due to its static structure, but it is only suitable for small changes. Thus, ISAM is usually applied in databases which are not frequently updated. Static index allocation makes frequent insertion and deletion an expensive process. However, it helps with concurrent access of records, which helps to scale the efficiency of the current data structure. The drawbacks of ISAM with regard to insertion/deletion was later addressed by B-tree based indexing. The ISAM approach was used before the adoption of tree-based indexing:

Figure 7.5: Illustration of ISAM structure

The ISAM data structure consists of a memory-resident cylinder index...