ABCs of z/OS System Programming Volume 3 - IBM Redbooks

DASD virtual visibility
Traditional S/390 architecture does not allow more than one I/O operation to the same S/390
device because such devices can only handle, physically, one I/O operation at a time.
However, in modern DASD subsystems such as ESS, DS6000, and DS8000, the device
(such as a 3390) is only a logical view. The contents of this logical device are spread in HDA
RAID arrays and in caches. Therefore, it is technically possible to have more than one I/O
operation towards the same logical device. Changes have been made in z/OS (in IOS code),
in the channel subsystem (SAP), and in ESS, DS6000, and DS8000 to allow more than one
I/O operation on the same logical device. This is called parallel I/O, and it is available in two
types:
► Multiple allegiance
► Parallel access volume (PAV)
This relief builds upon prior technologies that were implemented in part to help reduce the
pressure on running out of device numbers. These include PAV and HyperPAV. PAV alias
UCBs can be placed in an alternate subchannel set (z9® multiple subchannel support).
HyperPAV reduces the number of alias UCBs over traditional PAVs and provides the I/O
throughput required.
Multiple allegiance
Multiple allegiance (MA) was introduced to alleviate the following constraint. It allows
serialization on a limited amount of data within a given DASD volume, which leads to the
possibility of having several (non-overlapping) serializations held at the same time on the
same DASD volume. This is a useful mechanism on which any extension of the DASD volume
addressing scheme can rely. In other terms, multiple allegiance provides finer (than
RESERVE/RELEASE) granularity for serializing data on a volume. It gives the capability to
support I/O requests from multiple systems, one per system, to be concurrently active against
the same logical volume, if they do not conflict with each other. Conflicts occur when two or
more I/O requests require access to overlapping extents (an extent is a contiguous range of
tracks) on the volume, and at least one of the I/O requests involves writing of data.
Requests involving writing of data can execute concurrently with other requests as long as
they operate on non-overlapping extents on the volume. Conflicting requests are internally
queued in the DS8000. Read requests can always execute concurrently regardless of their
extents. Without the MA capability, DS8000 generates a busy indication for the volume
whenever one of the systems issues a request against the volume, thereby causing the I/O
requests to be queued within the channel subsystem (CSS). However, this concurrency can
be achieved as long as no data accessed by one channel program can be altered through the
actions of another channel program.
Parallel access volume
Parallel access volume (PAV) allows concurrent I/Os to originate from the same z/OS image.
Using PAV can provide significant performance enhancements in IBM System z environments
by enabling simultaneous processing for multiple I/O operations to the same logical volume.
PAV was introduced in z/OS V1R6 as a new option that allows to specify PAV capability as
one of the volume selection criteria for SMS-managed data sets assigned to a storage class.
HyperPAV feature
With the IBM System Storage DS8000 Turbo model and the IBM server synergy feature,
the HyperPAV together with PAV, multiple allegiance can dramatically improve performance
and efficiency for System z environments. With HyperPAV technology:
► z/OS uses a pool of UCB aliases.
58 ABCs of z/OS System Programming Volume 3