The common recall queue (CRQ) is a single recall queue that is
shared by multiple DFSMShsm hosts. This queue is implemented through
the use of a coupling facility (CF) list structure. The CRQ balances
the recall workload across multiple address spaces and enables optimization
for priority and single-tape mounts. In some parallel sysplex configurations,
all hosts are not connected to all devices. The CRQ enables hosts
that are not connected to a device, like a tape drive, to place recall
requests on the CRQ. Once on the CRQ, the request can be selected
by hosts that are connected to the device.
Figure 1. The Common Recall Queue
As Figure 1 shows, a CRQplex consists of one or more DFSMShsm hosts that are connected to the same CRQ. There may be multiple CRQplexes within a single HSMplex. In general, each HSMplex maintains a unique CRQ.
This CRQ support provides the following enhancements to recall
processing:
- Workload Balancing
- Alternating with all other participating hosts in the CRQplex, each host attempts to select requests from the CRQ. Each host selects requests while it has available recall tasks and the CRQ is not empty.
- Priority Optimization
- Because all recall requests are placed and sorted on a common queue, the highest priority outstanding requests in the CRQplex are selected before lower priority requests. This prioritization occurs regardless of which system the requests originated on.
- Tape Mount Optimization
- When a recall task mounts an ML2 tape to perform a recall, that task processes all outstanding single tape recall requests in the CRQ that require that tape. Thus, a single tape mount recalls multiple recall requests that require the same tape, even if those requests originated on different hosts.
- Request Persistence
- Outstanding recall requests on the CRQ are maintained even when the originating DFSMShsm host shuts down or fails, or the originating system fails. Additionally, those requests are eligible for processing by other active hosts, even while the original host is unavailable.
- Flexible Configurations
- The CRQ support enables configurations in which not all hosts are connected to all devices, and in which not all hosts process recalls. For example, you can configure a host that is not connected to any tape drives to process only recalls that do not require ML2 tapes. Also, you can configure the HSMplex so that all hosts accept recall requests and place them on the CRQ, but only a subset of the hosts actually process the requests.
Related reading
For more information about how to configure each host in an HSMplex for a CRQ environment, see z/OS DFSMShsm Implementation and Customization Guide.