StorageTek 312564001 manual Processing requests for access to migrated rows, SQL Select processing

Processing requests for access to migrated rows

Once DB2 Manager initialization has been completed, batch, TSO, CICS or IMS programs will be able to access migrated rows using standard SQL processing, without modification.

DB2 Manager implements support for access to migrated rows which is made via SQL SELECT, UPDATE and DELETE commands. Note that any rows created in a table via an SQL INSERT command will be stored in DB2. It is not possible to directly store a row in an Archive Manager database – a row must be migrated from a DB2 table to an Archive Manager database, using the DB2 Manager table migration utility.

SQL SELECT processing

Any reference to a migrated row generated via an SQL SELECT command will cause the DB2 Manager SQL intercept processor to pass a request to the DB2 Manager control region. This request will be processed by a scheduler task, which will perform some initial validation.

If the object containing the required row is disk-resident, the scheduler will pass the request to the disk reader task with the shortest request queue.

If the object containing the required row is tape-resident only, the scheduler will pass the request to the most appropriate tape reader task. This is selected using the following hierarchy:

1.A busy task which is processing the tape volume containing the object to be retrieved, and whose request queue is not greater than the maximum specified in the MAXQLEN initialization parameter.

2.A waiting task which has no tape mounted.

3.A waiting task which currently has a tape mounted.

	If there is no tape reader task satisfying any of the above conditions, the
	request will be rejected with a “resource unavailable” condition, unless a non-
	zero value is specified for the TAPEWAIT initialization parameter. In these
	circumstances, the request is queued internally and retried until it has been
	successfully assigned to a tape reader task, or until the time interval specified
	in the TAPEWAIT parameter has been exceeded, in which case it will be
	rejected with a “resource unavailable” condition. Refer to page 41 for a
	description of the TAPEWAIT parameter.
	When a request has been passed to a tape or disk reader task, the task will
	locate the object containing the migrated row and extract the logical record
	containing the row. This will be passed back to the SQL intercept routine,
	which will in turn pass the required row back to the calling application.
	The RETAINTAPE initialization parameter may be used to retain a tape
	volume on a drive for a specified period of time when a tape reader task has
	processed all outstanding requests for retrieval of objects from that tape
48	DB2 Manager User Guide
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