Step 7. Space for Cylinder Indexes and Track Indexes

You will need space for a cylinder index and track indexes. There is a cylinder index entry for each track index (for each cylinder allocated for the data set). The size of each entry is the same as the size of the track index entries; therefore, the number of entries that will fit on a track is the same as the number of track index entries. Unused space on a cylinder index track is not shared.

   Number of tracks  = (Track indexes + 1)
   required for      / (Index entries per track cylinder index)

Example: You have 17 track indexes (from Step 6). Because 57 index entries fit on a track (from Step 3), you need 1 track for your cylinder index. The remaining space on the track is unused.

Number of tracks required for cyl. index = (17 + 1) / 57 = 18 / 57 = 0.316 < 1

Every time a cylinder index crosses a cylinder boundary, ISAM writes a dummy index entry that lets ISAM chain the index levels together. The addition of dummy entries can increase the number of tracks required for a given index level. To determine how many dummy entries will be required, divide the total number of tracks required by the number of tracks on a cylinder. If the remainder is 0, subtract 1 from the quotient. If the corrected quotient is not 0, calculate the number of tracks these dummy entries require. Also consider any additional cylinder boundaries crossed by the addition of these tracks and by any track indexes starting and stopping within a cylinder.