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

3.3 zArchitecture data scalability
Serialization
Granularity
reserve
Access
visibility
Physical
volume
release
3390-3 3390-9
Figure 3-3 zArchitecture data scalability
Multiple allegiance .......................................
Base UCB
3390-9
Parallel Access Volume (PAV)
HyperPAV
Alias UCBs
Dynamic volume
expansion
3390-9
"3390-54"
3 GB 9 GB 27 GB 54 GB
max cyls: 3339 max cyls: 10017 max cyls: 32760 max cyls: 65520
DASD architecture
In the past decade, as processing power has dramatically increased, great care and
appropriate solutions have been deployed so that the amount of data that is directly
accessible can be kept proportionally equivalent. Over the years DASD volumes have
increased in size by increasing the number of cylinders and thus GB capacity.
years
However, the existing track addressing architecture has limited growth to relatively small GB
capacity volumes. This has placed increasing strain on the 4-digit device number limit and the
number of UCBs that can be defined. The largest available volume is one with 65,520
cylinders or approximately 54 GB, as shown in Figure 3-3.
Rapid data growth on the z/OS platform is leading to a critical problem for various clients, with
a 37% compound rate of disk storage growth between 1996 and 2007. The result is that this
is becoming a real constraint on growing data on z/OS. Business resilience solutions
(GDPS®, HyperSwap®, and PPRC) that provide continuous availability are also driving this
constraint.
Serialization granularity
Since the 1960s, shared DASD can be serialized through a sequence of
RESERVE/RELEASE CCWs that are today under the control of GRS, as shown in Figure 3-3.
This was a useful mechanism as long as the volume of data so serialized (the granularity)
was not too great. But whenever such a device grew to contain too much data, bottlenecks
became an issue.
Chapter 3. Extended access volumes 57