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DB2 Manager User Guide 11
StorageTek Proprietary
A migrated row which has been retrieved from an Archive Manager database
is presented back to the calling application on reference via SQL. No
indication is given to the application that the row has been retrieved from
Archive Manager, and the contents of the row will be identical to those prior
to migration of the row from DB2.
The following factors will govern the length of time taken to access a
migrated row stored in a tape-resident object in an Archive Manager
database:
- The time taken to mount a tape. This is dependent on the automatic
library robot accessor utilization rate.
- The time taken to locate an object on a tape once mounted. This will
depend on the amount of data held per Archive Manager tape volume
(user-controlled via Archive Manager database initialization
parameters).
- Tape drive availability. Lack of tape drives will prevent immediate
recall of objects from tape.
DB2 Manager provides the following controls to optimize tape recall
performance:
- Specification of the maximum number of tape drives to be used by
DB2 Manager for object retrieval. These controls are similar to those
provided by the online recall component of Archive Manager (ie)
drives required by DB2 Manager for tape object retrieval will be
dynamically allocated and released as required, up to the maximum
specified in the DB2 Manager drive control parameters.
- Parameterized controls are provided to allow users to limit the length
of queues of retrieval requests for any one specific tape volume. All
requests subsequent to the first for a mounted tape will be satisfied by
repositioning the tape. However, it may take up to 30 seconds to
reposition a tape; in order to provide a control over the length of time
taken to retrieve any one object, a limit may be placed on the number
of requests queued for an active tape.
If insufficient resources are available to satisfy an object retrieval request,
DB2 Manager will reject the request with the appropriate SQL error and
reason codes, or may optionally queue the request internally until sufficient
resources become available or until a customer-specified time interval has
elapsed.
The above controls are specified using the DB2 Manager parameter library,
and are processed during control region initialization. They may be
dynamically adjusted during DB2 Manager operation via control region
command processing.
Update of migrated rows
If an application updates a migrated row after retrieval from an Archive
Manager database, the migrated copy of the row will be invalidated and the