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48 DB2 Manager User Guide
StorageTek Proprietary
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