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Step 7. Space for Cylinder Indexes and Track Indexes z/OS DFSMS Using Data Sets SC23-6855-00 |
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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.
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.
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. |
Copyright IBM Corporation 1990, 2014
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