With the system-managed tape library support, each
scratch allocation (through the ACS routines) will assign one or more
tape storage groups and each tape storage group can be associated
with 1 to 8 tape libraries. As a result of this, there can be many
tape subsystems (and devices) that are considered eligible for each
scratch request. A limitation exists today on the number of tape subsystems
(device pools) that SMS and MVS allocation can support on a scratch
allocation request. Prior to z/OS V2R2, this limit was 253 tape subsystems
or 4048 devices (253 x 16 devices). With z/OS V2R2 or later (also
z/OS V1R13 and V2R1 with SMS APAR OA44354), this limit can be doubled
to 506 tape subsystems or 8096 devices (506 x 16 devices) for JES2
or JES3 managed devices by using the DEVSUPxx enablement indicator
GREATER_253. If GREATER_253=YES is specified, SMS will detect and
limit the number of eligible tape subsystems (device pools) to the
first 506 subsystems (returned by OAM) and will ignore the tape subsystems
beyond 506. By default, this enablement indicator is disabled (GREATER_253=NO).
The DEVSUP PARMLIB member can be updated outside of an IPL using
the SET DEVSUP=xx operator command.
Before enabling the GREATER_253
indicator, ensure that all systems in the sysplex can support the
increased limit (starting with z/OS V2R2, or z/OS V1R13 and V2R1 with
SMS APAR OA44354). If GREATER_253=YES is specified and some systems
in the sysplex do not have the required support installed, job abends
could occur.If an installation is using the TS7700 Virtualization
Engine and assigns SGTAPE to the allocation request and SGTAPE is
associated with several multi-cluster grids, the number of eligible
subsystems can be quite large.
As a result, having multiple grids and clusters within the grid being
eligible for a scratch allocation request could grow beyond the previous
subsystem limit of 253. If there are more than 506 tape subsystems
that are considered eligible for the scratch request, SMS will only
return the first 506 tape subsystems to MVS allocation. As you start
using the TS7700 Virtualization Engine and have more and more tape
subsystems being eligible, review your ACS routines and the number
of tape libraries, and subsystems, being used for your scratch allocations
to ensure that you fall within the 506 tape subsystem limit and that
you have the GREATER_253=YES enablement indicator specified as appropriate.
It is important to note that tape subsystems that are connected and
offline are still counted towards this subsystem limit. The main eligibility
criteria being whether we are able to talk to the device to understand
its characteristics and the characteristics of the device matching
what is requested on the allocation request.