Estimating Space Requirements for the BCS

The amount of space a BCS requires depends on the type and number of data sets and objects cataloged in it. The amount of space in the BCS used by each type of data set or object can vary, depending on the:

Length of the data set or component names
Number of volumes per data set
Number of relationships between components
Number of alternate indexes for a data set
Number of paths
Presence of security information
Presence of Storage Management Subsystem information.

Table 1 lists the various types of data sets and objects, and approximates the number of bytes needed in the BCS to maintain information about a data set or object of that type. The values do not necessarily represent the length of any one record.

Data sets used by NFS or DFM will have 455 bytes added to their published size to accommodate new cell types.

Table 1. Estimated Space Needed for Each Type of Data Set or Object
Data Set or Object Type	Number of Bytes
generation data group	350
generation data set	200
alias entry (see note)	150
non-VSAM data set or OAM object collection	200
user catalog connector (see note)	200
VSAM entry-sequenced, linear, or relative record data sets	400
VSAM key-sequenced data set	650
alternate index	400
path	190

Note: To simplify the calculation for a user catalog connector record and to account for all of the catalog's aliases, count each catalog connector as requiring 32400 bytes.

Use the following steps to estimate the space allocation for a BCS:

Estimate the number of each type of data set or object that will be cataloged in the BCS. Using these figures, determine the total space requirement, in bytes, for the BCS. This figure is the minimum amount of space that the BCS requires.
Increase this quantity by the amount of free space required, and add additional space to allow for growth and any inaccuracies in the calculation. For example, if you define free space as 20% of each control interval and area, multiply by 1.7.
Divide the total number of bytes by 1024 to determine the number of kilobytes, or by 1048576 to determine the number of megabytes. Round the result up to the nearest whole integer, and specify KILOBYTES or MEGABYTES as appropriate.
Choose an appropriate secondary allocation. It is best if the secondary allocation is larger than the equivalent of one cylinder, so that the control area is defined as one cylinder.

The number of kilobytes or megabytes in a control area of one cylinder for different IBM® DASD devices can be determined by multiplying the number of tracks per cylinder by the track capacity, and dividing by 1024 (kilobytes) or 1048576 (megabytes).

If you want to allocate space in tracks or cylinders, you must perform some additional calculations:

Divide the total number of bytes (including free space) by the data control interval size. Round up to the nearest whole integer. This is the number of required data control intervals.
Determine the number of control areas required by dividing the number of required control intervals by the number of control intervals that fit into your selected control area. Round up to the nearest integer.
Specify TRACKS or CYLINDERS as appropriate. If you specify TRACKS, space can be allocated in cylinders if the number of tracks is more than one cylinder.