The control interval sizes you specify when the data set is defined
are not necessarily the ones that appear in the catalog. VSAM makes
adjustments, if possible, so that control interval size conforms to
proper size limits, minimum buffer space, adequate index-to-data size,
and record size. VSAM makes the following adjustments when your
data set is defined.
- Specifies data and index control interval size. After VSAM determines
the number of control intervals in a control area, it estimates whether
one index record is large enough to handle all control intervals in
the control area. If not, the size of the index control interval is
increased, if possible. If the size cannot be increased, VSAM decreases
the number of control intervals in the control area.
- Specifies maximum record size as 2560 and data control interval
size as 2560, and have no spanned records. VSAM adjusts the data control
interval size to 3072 to permit space for control information in the
data control interval.
- Specifies buffer space as 4K, index control interval size as 512,
and data control interval size as 2K. VSAM decreases the data control
interval to 1536. Buffer space must include space for two data control
intervals and one index control interval at DEFINE time. For more
information about buffer space requirements see Determining I/O Buffer Space for Nonshared Resource.