Consider the following when estimating the space needed for a VVDS:

• If you want to allocate space in tracks, you must divide the total number of bytes by 4096 (the control interval for a VVDS). Then, divide the result by the number of 4096 blocks per track for the device.
• Track allocations may be rounded up to the next cylinder value. For example, a define request for 5455 tracks will likely allocate 5460 tracks, which is evenly divisible by 15 and is the next cylinder boundary.
• The VVDS can hold a maximum of 1048575 control intervals (CIs). This limits defining of primary and secondary allocation sizes to 87375 tracks or 5825 cylinders or fewer. This limit is also in effect when extending the VVDS. Thus, if extending the VVDS by the secondary allocation amount causes it to exceed the CI limit, the extend will fail. If you know the average number of VVRs per CI in the VVDS, you can estimate the maximum number of data sets the VVDS can support on the volume (IDCAMS PRINT of the VVDS can assist in coming to an estimate of average VVR's per CI).
• You can explicitly create a VVDS that can optionally be placed in Extended Addressable Storage (EAS), if available. Allocation amounts which are intended for EAS space may be rounded up to a Multi-Cylinder Unit (MCU). The potential rounding up of EATTR(OPT) allocations has the effect of lowering the VVDS maximum size to CI values which are consistent with the MCU boundary.
• Explicit definitions of a VVDS with EATTR(OPT) above the last usable MCU while under the VVDS CI maximum will succeed, but the allocation will be rounded down to the last usable MCU boundary. Additionally, extending of the VVDS which was both explicitly defined and EATTR(OPT) will fail, if extending the VVDS will exceed the last usable MCU boundary prior to the VVDS CI maximum.