StorageTek Host Software Component 6.0, System Programmer'S Manual

Page: 419 / 718

Chapter 5. Problem Determination, Diagnostics, and Recovery 389
1st ed., 6/30/04 - 312579601
Switching to a Backup Copy of the Control Data Set
If you have initialized your HSC subsystem with the additional backup copies of the
control data set, you can dynamically switch operation to any one of the copies without
affecting the HSC and library operation.
With an operator command, you can enable or disable specific copies of the control data
set. When a copy of the CDS is disabled, the specified control data set is taken offline
while any of the backup control data sets are brought online; the HSC continues to run the
the library installation during the switch. When a new control data set is brought online, all
work within a library complex needing the control data set halts until the switch function
completes. Depending on the size of the complex, this switching time can take a few
minutes. However, this is a better alternative than an HSC shutdown and subsequent
restore of data sets, especially in a multi-host environment.
Operator commands are available to display the status of current control data sets or to
enable/disable/autoenable any copy of the control data set. Refer to the
HSC Operator’s
Guide for detailed information about the commands.
Switching the Control Data Set in a Multi-Host Environment
In a multi-host environment, the HSC automatically provides communications to all hosts
when a control data set switch has occurred. For this communications performance to
occur, the proper hardware and software must exist in the library installation and the
parameter data set must specify parameters invoking utilization of the hardware and
software.
Control Data Set Integrity During Restore
To take advantage of the recovery capabilities of the HSC during the restore operations, it
is necessary to ensure that you have initialized a standby control data set.
Refer to Chapter 7, ‘‘Allocating and Initializing Control Data Sets’’ in the HSC
Installation Guide for detailed information about installing the HSC and initializing
control data sets.
Detecting Mismatch of Control Data Sets
Control data set mismatch occurs when the HSC has determined that the primary and
secondary (or shadow) control data sets are not synchronized. Normally, the HSC
automatically determines which copy of the control data set is valid and switches
operation to that data set.
The recovery performed by the HSC is transparent to you, except for the issuance of
appropriate messages to the operator. Through the messages, the operator is made aware
of the switching that has occurred.
In cases where the HSC cannot determine which control data set is valid, you must run the
BACKup and RESTore utilities to recover the data sets. Refer to “When CDS Copies Are
Split Among Hosts After an Error” on page 213 for some special considerations
concerning control data sets.