HVTOC - High Capacity VTOC
HVTOC, or High Capacity VTOC, is an enhanced version of the Volume Table of Contents specifically designed to support the significantly larger storage capacities of Extended Address Volumes (EAVs) on IBM z/OS systems. It enables the management of a much greater number of data set entries than a traditional VTOC, accommodating the vast number of data sets that can reside on a single EAV.
Key Characteristics
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- Increased Capacity: HVTOCs can hold millions of data set entries, far exceeding the approximately 262,000 entries of a traditional VTOC, making them suitable for
EAVs. - EAV Requirement: HVTOCs are mandatory for
Extended Address Volumes (EAVs)to manage the extensive addressable space and numerous data sets these volumes can contain. - VTOC Index: Unlike traditional VTOCs, HVTOCs always include a
VTOC index, which is a VSAM Key-Sequenced Data Set (KSDS) that provides efficient access to data set control blocks (DSCBs). - Location: An HVTOC resides on the
EAVitself, typically at the beginning of the volume, similar to a traditional VTOC. - Dynamic Growth: The VTOC index can dynamically grow as more data sets are allocated on the volume, ensuring scalability without requiring manual intervention to extend the VTOC area.
- DFSMS Managed: HVTOCs are managed by
DFSMS(Data Facility Storage Management Subsystem) components, ensuring proper allocation, deallocation, and integrity.
- Increased Capacity: HVTOCs can hold millions of data set entries, far exceeding the approximately 262,000 entries of a traditional VTOC, making them suitable for
Use Cases
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- Managing EAVs: The primary use case is to enable and manage data sets on
Extended Address Volumes, which provide terabytes of storage capacity on a single logical volume. - Consolidating Data: Organizations can consolidate a large number of smaller data sets onto fewer, larger EAVs, reducing the number of physical volumes and simplifying storage management.
- High Data Set Density: Environments requiring a very high density of data sets on a single volume, such as those used for development, testing, or specific application data, benefit from HVTOCs.
- Modern Storage Infrastructure: Essential for leveraging modern IBM DASD hardware and
DS8000series storage controllers that support EAVs and require their advanced capabilities.
- Managing EAVs: The primary use case is to enable and manage data sets on
Related Concepts
HVTOC is a direct evolution of the traditional VTOC (Volume Table of Contents), which it supersedes in functionality and capacity for EAV volumes. It works in conjunction with the VVDS (VSAM Volume Data Set), which stores attributes for VSAM data sets and SMS-managed non-VSAM data sets, with the VTOC index pointing to both data set control blocks (DSCBs) and VVDS entries. HVTOCs are fundamental to DFSMS (Data Facility Storage Management Subsystem) for managing storage on EAVs, allowing SMS (Storage Management Subsystem) to effectively allocate and track data sets across these high-capacity volumes.
- Plan EAV Implementation: Carefully plan the adoption of
EAVsand HVTOCs, considering the potential for a massive number of data sets and the associated management implications for backup, recovery, and performance. - Monitor VTOC Index Growth: Regularly monitor the growth and utilization of the
VTOC indexto ensure it has sufficient free space and