Backup while open (BWO) of VSAM files

CICS® supports the backup while open (BWO) facility provided by DFSMSdss and DFSMShsm. This support enables some types of VSAM data sets to be backed up by DFSMSdss while CICS is currently updating these data sets. At the same time, CICS logs forward recovery images of any changes to these data sets on a forward recovery journal. At a later date the backup of the data set can be restored using DSMShsm and brought to a point of consistency by applying the forward recovery logs using a forward recovery utility such as the CICS VSAM Recovery (CICSVR).

BWO is available only for data sets accessed by CICS file control, which includes the CICS system definition (CSD) data set.

VSAM data sets that are to use this facility must reside on SMS-managed DASD, and must have an ICF catalog structure. Only VSAM ESDS, RRDS (both fixed and variable), and KSDS data sets are supported.

For DFSMS 1.3, there are two ways of defining BWO:

  1. By defining the cluster with parameter BWO. This parameter can take the values TYPECICS, TYPEIMS, TYPEOTHER, and NO. TYPECICS means that it is eligible for BWO in a CICS region. The other parameters are treated as not eligible. The file resource definition is ignored (even if it conflicts).
  2. If the BWO parameter is not defined, it defaults to UNDEFINED. In this case CICS looks at the file resource definition.

Clusters with data sets that are to be opened in RLS mode must have BWO specified in the cluster definition.

CICS defines a data set as eligible for BWO when a file is defined using RDO. If BACKUPTYPE=DYNAMIC is specified for a VSAM file, the file is defined as eligible for BWO when the data set is opened. BACKUPTYPE=STATIC, the default, defines a file as not eligible for BWO.

If DFSMSdss is to back up a data set that is specified with BACKUPTYPE=STATIC, all CICS files currently open for update against that data set must be closed before the backup can start.

The first time a file is opened against a VSAM base cluster data set after a CICS initial or cold start, CICS checks if BWO has been specified in the ICF catalog. If it is, it updates information in the file resource definition from the ICF catalog.

If the data sets are updated in RLS mode, BWO is managed entirely by DFSMS. When a BWO copy is made, DFSMSdss sends a message to all CICS systems on the sysplex with open ACBs for the sphere. The CICS systems keep track of all current UOWs that have updated files for the sphere. When all of these have completed, CICS writes tie-up records and notifies DFSMSdss. The copy is complete when all CICS systems have responded.

If the data sets are updated in non-RLS mode and if the value specified by BACKUPTYPE is DYNAMIC, CICS issues a call to DFSMSdfp 3.2 callable services to update the ICF catalog to indicate that the base cluster data set is eligible for BWO while it is under the control of CICS.

Any subsequent file opened against the same cluster must have the same BACKUPTYPE attribute as that of the first file opened. If a mismatch is found, the subsequent file open fails.

CICS records the fact that a VSAM base cluster data set is eligible for BWO in its base cluster block. This is remembered when all files have closed against the VSAM base cluster and across CICS warm and emergency restarts. (It is not remembered across CICS cold or initial starts.) When CICS is terminated by a controlled normal shutdown, all CICS files are closed.

When the last file open for update (and defined as eligible for BWO) is closed against a base cluster data set, the DFSMSdfp callable services update the ICF catalog to indicate that this data set is no longer eligible for BWO. This prevents BWO during the batch window between CICS sessions.

Note:
During the batch window between CICS sessions it is possible to update CICS user data sets by batch jobs (although, to maintain data integrity, this should only be done after a controlled normal shutdown, and never after an uncontrolled or immediate shutdown).

Any BWO backup made during a batch window after an uncontrolled or immediate shutdown should be discarded if batch updates are made. This is because those updates are not logged to CICS forward recovery logs and therefore the BWO backup could not be forward recovered to a point of consistency.

For a normal CICS shutdown, CICS also needs a quiesced data set 5 backup to be made after the batch updates and before the data set is made available to a subsequent CICS session so that CICS forward recovery can start from a consistent point.

Before DFSMSdss and DFSMShsm take a backup of any kind of a VSAM sphere, a call is made to examine the state of the ICF catalog to check if BWO is required. If so, the backup is made without attempting to obtain exclusive control and serialize updates to this data set.

When a backup copy of a data set is restored by DFHSM and DFDSS, and the backup was of a BWO type, the ICF catalog is updated to indicate that the data set needs to be forward recovered before it can be used. CICS checks this at data set open time and fails an FCT open if the catalog indicates that the data set is back-level.

The systems administrator must put appropriate procedures into place for BWO and for forward recovery, but these new procedures should be simpler than those currently in use. These procedures must include:

The systems administrator must decide which VSAM user data sets are eligible for BWO, subject to the restrictions detailed in Restrictions on BWO applicable to heavily-updated KSDS data sets.

Effect of disabling activity keypointing

Note:
This affects only non-RLS activities.

If activity keypointing is disabled in your CICS region (by specifying the system initialization parameter AKPFREQ=0), this has a serious effect on BWO support, because no tie-up records (TURs) are written to the forward recovery logs, and the data set recovery point is not updated. Therefore, forward recovery of a BWO backup must take place from the time that the data set was first opened for update. This requires that all forward recovery logs are kept since that time so that forward recovery can take place. If there are many inserts or records that change length, a lot of forward recovery could be required. If, however, a record is just updated and the length is unchanged, there is no CI split. For information about TURs and recovery points, see the CICS Recovery and Restart Guide.

Restrictions on BWO

The following restrictions apply to VSAM KSDS data set types only.

If a VSAM control interval or control area split occurs while a BWO is in progress, the backup is unreliable and is discarded by DFHSM and DFDSS. During such a split, certain portions of the data set may be duplicated or not represented at all in the backup as DFDSS copies sequentially. MVS/DFP 3.2 indicates that a split has occurred in the ICF catalog. At the end of the backup, DFHSM and DFDSS check the ICF catalog, and if a split has occurred, or is still in progress, discard the backup. For this reason, certain heavily-updated VSAM KSDS data sets may not be eligible for BWO, or might be eligible only during periods of reduced activity (for example, overnight). For a KSDS data set to be eligible for BWO, the typical time between control interval or control area splits must be greater than the time taken for DFHSM and DFDSS to take a backup of the data set.

XRF considerations

CICS XRF regions take keypoints more frequently than non-XRF regions. If BWO is used, extra activity occurs during keypoints, because:

These actions are performed approximately once every 30 minutes.


5.
Quiesced data set: A data set against which all update activity has been quiesced so that DFSMSdss can have exclusive control while a backup is made.

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