CA Dynam for VSE User Guide

Managing the VTOC
Searching the VTOC
A request for a format-1 record would normally result in a hardware search
(on CKD devices) of the entire VTOC, or a sequential reading (on FBA devices)
of the VTOC.
With the presence of a VTOC index, the following search method is used:
• Looking up the data set name in the VTOC index (usually a single I/O)
• Reading the format-1 record directly from the VTOC (a single, short I/O)
Thus the channel is freed for other work, and I/O to the VTOC is minimized.
The index to the VTOC is built as a prefix-balanced tree structure. A balanced
tree structure is one in which an equal number of index records must be
searched (one record for each index level) for all entries in the index. A prefixbalanced
tree, or prefix B-tree, is a balanced tree in which common portions of
keys (in this case, data set names) are stored only on higher index levels,
conserving space on lower levels.
A VTOC index also contains a record of all space on the volume, depending on
whether it is occupied. This information is used to allocate new data sets and
check for overlapping extents, greatly reducing the time (and resources) spent
performing these functions.
Creating New VTOC Records
When a new VTOC record is to be created, a free (format-0) VTOC record must
be found. This operation is usually similar to the format-1 record lookup
described above, except that a search is made for a record having a key of
binary zeros. The process is subject to the same problems as the format-1
lookup described above.
In order to alleviate this situation, a VTOC index contains a bit map describing
the VTOC itself. When free VTOC records are required, they are found by
examining the VTOC allocation map, instead of the conventional search.
4–12 User Guide