When allocating space for a new data set on DASD, SMS converts all space requests in tracks (TRK) or cylinders (CYL) into requests for space in KB or MB. If a generic device type such as the 3380 is specified, SMS uses the device geometry for that generic device to convert tracks or cylinders into KB or MB. If an esoteric device type such as SYSDA or no UNIT is specified, SMS uses the default device geometry to convert tracks and cylinders into KB or MB. If the users in your installations specify space in tracks or cylinder units, and they specify an esoteric UNIT or no UNIT, you must specify a default device geometry prior to converting these allocations to system-managed data sets.

After SMS converts space requests to KB or MB, the space values are passed to the ACS routines. The values are later used to determine the number of tracks or cylinders to allocate for the data set. The default device geometry does not apply to objects or data sets allocated on tape.

There is only one default device geometry for the entire SMS complex. Default device geometry is an installation's definition of how much space is represented by a TRK or a CYL when an esoteric unit or no unit is specified. The device geometry is the track size and number of tracks per cylinder for the device.

The device geometry for 3380 is 47476 bytes per track, 15 tracks per cylinder. The device geometry for 3390 is 56664 bytes per track, 15 tracks per cylinder. It is up to each installation to decide what values to use.