Extended Address Volumes

An extended address volume is a volume with more than 65 520 cylinders. An extended address volume increases the amount of addressable DASD storage per volume beyond 65 520 cylinders by changing how tracks on ECKD volumes are addressed.

A track address is a 32-bit number that identifies each track within a volume. The address is in the format hexadecimal CCCCcccH.

CCCC is the low order 16-bits of the cylinder number.
ccc is the high order 12-bits of the cylinder number.
H is the four-bit track number.

The combination of the 16-bits and 12-bits for the low order and high order cylinder number represents a 28-bit cylinder number. IBM recommends the use of the TRKADDR macro for the manipulation of track addresses. See z/OS DFSMSdfp Advanced Services for more information.

For an extended address volume, the extended addressing space (EAS) is cylinders whose addresses are equal to or greater than 65,536. The ccc portion is non-zero for the cylinders of EAS. These cylinder addresses are represented by 28-bit cylinder numbers.

For compatibility with older programs, the ccc portion is hexadecimal 000 for tracks in cylinders whose addresses are below 65,536. These cylinder addresses are represented by 16-bit cylinder numbers. This is the base addressing space on an extended address volume.

A multi-cylinder unit is a fixed unit of disk space that is larger than a cylinder. Currently, on an EAV, a multicylinder unit is 21 cylinders and the number of the first cylinder in each multi-cylinder unit is a multiple of 21.

The cylinder-managed space is space on the volume that is managed only in multicylinder units. Cylinder-managed space begins at cylinder address 65,520. Each data set occupies an integral multiple of multicylinder units. Space requests targeted for the cylinder-managed space are rounded up to the next multicylinder unit. The cylinder-managed space only exists on EAV volumes.

The track-managed space is space on a volume that is managed in tracks and cylinders. Track-managed space ends at cylinder address 65,519. Each data set occupies an integral multiple of tracks. Track-managed space also exists on all volumes.

For an extended address volume, the system and storage group break point value (BPV) helps direct disk space requests to cylinder or track-managed space. The breakpoint value is expressed in cylinders. When the size of a disk space request is the breakpoint value or more, the system prefers to use the cylinder-managed space for that extent. This rule applies to each request for primary or secondary space for data sets that are eligible for the cylinder-managed space. If cylinder-managed space is insufficient, the system uses the track-managed space or uses both types of spaces. When the size of a disk space request is less than the breakpoint value, the system prefers to use the track-managed space. If space is insufficient, the system uses the cylinder-managed space or uses both types of spaces.

Almost all types of data sets are EAS-eligible, including the following:

SMS and non-SMS managed VSAM data sets (all types), including:
- BCS and VVDS catalog data sets
- VSAM data sets inherited from prior physical migrations or copies
- VSAM temporary data sets
zFS data sets (they are VSAM).
Sequential data sets, including extended, basic, and large formats
PDS and PDSE data sets
Direct (BDAM) data sets
Data sets allocated with undefined DSORGs

You can control whether EAS-eligible data sets can reside in cylinder-managed space by:

Including or excluding EAVs in particular storage groups.
For non-SMS managed data sets, controlling the allocation to a volume by specifying a specific VOLSER or esoteric name.
Using the new EATTR data set attribute keyword to specify that the data set supports extended attributes (format 8 and 9) and can reside in the EAS of an EAV.

Non-EAS eligible data sets include:

HFS data sets
Page data sets
VTOC and VTOC index data sets
VSAM data sets with imbed or keyrange attributes that may have been inherited from prior physical migrations or copies

Only VSAM data sets that are allocated with compatible Control Areas (CA), for non-striped VSAM, and Minimum Allocation Units (MAU), for striped VSAM, can reside or be extended in cylinder-managed space. A compatible CA or MAU size are those that divide evenly into the multicylinder unit of value of cylinder-managed space.

The following CA sizes and MAU are compatible because they divide evenly into the multicylinder unit of 21 cylinders (315 tracks):

1, 3, 5, 7, 9, 15 Tracks

The system ensures for all new allocations on all volume types that a compatible CA or MAU is selected.

SMS storage groups with a mix of EAV and non-EAV volumes are supported. When building the volume candidate list, SMS prefers the following settings:

volumes based on the data set type
the size of the space request
the multicylinder unit size
the user’s defined break point value
the free space statistics from both managed spaces on the volume

Generally, for VSAM data set allocation requests that are equal to or larger than the BPV, SMS prefers EAV volumes. For non-VSAM allocation requests and VSAM allocation requests that are smaller than the BPV, EAV volumes are not preferred.

See z/OS DFSMSdfp Advanced Services for more information on the VTOC and INDEX structures of an extended address volumes.

See z/OS DFSMS Using the New Functions for the information about how to set up and use the EAV.