StorageTek 312564001 manual Use of the MAXDRDR parameter, Allocation recovery

Rejection of a request due to the MAXQLEN value having been reached or due to the TAPEWAIT interval having been exceeded will generate an SQL – 652 error with a reason code of X'1015' in field SQLERRD(6) in the SQLCA.

Use of the MAXDRDR parameter

The MAXDRDR parameter is used to control the number of disk reader tasks which are active in the DB2 Manager control region. A disk reader task is used to perform retrieval of a migrated row from an Archive Manager database when a disk (‘K’) copy exists of the object containing the row.

If the value of MAXDRDR is greater than 1, DB2 Manager will assign a new disk object retrieval request to the disk reader task with the shortest request queue. In this way, the length of the request queues will be balanced across the allocated disk reader tasks. Increasing the value of MAXDRDR will decrease the average disk reader task request queue length. However, there will be a corresponding increase in the amount of virtual storage required by the DB2 Manager control region. For most operational environments, the default MAXDRDR value of 4 will be sufficient.

If the value of MAXDRDR is 0, then all object retrieval requests, whether objects have a disk copy or not, will be processed by the tape reader tasks currently allocated in the control region. Each tape reader task will issue a standard Archive Manager object retrieval request; if a disk copy of an object exists then Archive Manager will satisfy the retrieval request from disk (i.e. the object will still be retrieved from disk, even though it is being processed by a tape reader task). Setting the MAXDRDR value to 0 is not recommended in these circumstances, as it may cause disk object retrievals to be rejected unnecessarily if there are no available tape reader tasks to process the request.

If an error occurs when Archive Manager attempts to retrieve the disk copy of an object, Archive Manager will automatically re-drive the retrieval attempt from tape. In these circumstances it is possible that a disk reader task will attempt to process a tape dataset. This may cause contention for tape drives, resulting in failure of the retrieval request, if all available drives are currently being processed by the tape reader tasks. Retrieval of the tape copy of an object by a disk reader task will not impact the ability of that task to continue accepting and processing further disk object retrieval requests.

Allocation recovery

Allocation recovery occurs on MVS systems when there are no tape drives available to satisfy an allocation request. In this case, the following message will be output by the system:

IEF238D jjj - REPLY DEVICE NAME, 'WAIT' OR 'CANCEL'

Normal practice is for an operator (or automated operator facility) to reply WAIT to this request. The job requiring the allocation will then wait until a drive becomes available before being able to continue.

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