Step 8. Space for Master Indexes

If you have a data set large enough to require master indexes, you will want to calculate the space required according to the number of tracks for master indexes (NTM parameter) you specified in the DCB macro or the DD statement.

If the cylinder index exceeds the NTM specification, an entry is made in the master index for each track of the cylinder index. If the master index itself exceeds the NTM specification, a second-level master index is started. As many as three levels of master indexes are created if required.

The space requirements for the master index are computed in the same way as those for the cylinder index.

Calculate the number of tracks for master indexes as follows:

# Master index tracks = (# Cylinder index tracks + 1) / Index entries per track

If the number of cylinder indexes is greater than NTM, calculate the number of tracks for a first level master index as follows:

   # Tracks for first level master index =
   (Cylinder track indexes + 1) / Index entries per track

If the number of first level master indexes is greater than NTM, calculate the number of tracks for a second level master index as follows:

   # Tracks for second level master index =
   (First level master index + 1) / Index entries per track

If the number of second level master indexes is greater than NTM, calculate the number of tracks for a third level master index as follows:

   # Tracks for second level master index =
   (Second level master index + 1) / Index entries per track

Example: Assume that your cylinder index will require 22 tracks. Because large keys are used, only 10 entries will fit on a track. If NTM was specified as 2, 3 tracks will be required for a master index, and two levels of master index will be created.

   Number of tracks required for master indexes = (22 + 1) / 10 = 2.3