As an optional feature, the operating system creates a master index at your request. The presence of this index makes long, serial searches through a large cylinder index unnecessary.
You can specify the conditions under which you want a master index created. For example, if you have specified NTM=3 and OPTCD=M in your DCB macro, a master index is created when the cylinder index exceeds 3 tracks. The master index consists of one entry for each track of cylinder index. If your data set is extremely large, a higher-level master index is created when the first-level master index exceeds three tracks. This higher-level master index consists of one entry for each track of the first-level master index. This procedure can be repeated for as many as three levels of master index.