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Hardware requirements
The concurrent copy function is supported by the IBM
®
3990 Model 3 with the
extended platform and the IBM 3990 Model 6 control units.
Which data sets are eligible for BWO
You can use BWO only for:
v Data sets that are on SMS-managed storage and that have an integrated catalog
facility (ICF) catalog.
v VSAM data sets accessed by CICS file control and for the CICS system definition
(CSD) file. ESDS, KSDS, and RRDS are supported. ESDS and KSDS are
supported both with and without alternate indexes (AIX). DFSMShsm imposes a
limit (many thousands) on the number of AIXs for a data set.
BWO is supported at the VSAM sphere level; thus you cannot take BWO backup
copies of some sphere components and not others. The first data set opened for
update against a VSAM base cluster determines the BWO eligibility for the sphere.
This includes base clusters that are accessed through a VSAM path key. For
example, if the first data set is defined as eligible for BWO, CICS fails the file-open
operation for any subsequent data set that is opened for update against that cluster
and which is not defined as eligible for BWO.
You can take BWO volume backups if all data sets that are open for update on the
volume are eligible for BWO.
VSAM control interval or control area split
For a KSDS (or an ESDS with AIX) that has frequent insert or update activity, it is
only practical (and safe) to take BWO backups during periods of reduced activity
(for example, overnight). This is because it is possible for a VSAM control interval
or control area split to occur during a BWO backup. Then, the integrity of the
backup copy cannot be guaranteed because DFSMSdss copies the data set
sequentially, and so certain portions of the data set might be duplicated or not
represented at all in the backup copy.
DFSMSdfp indicates in the ICF catalog that a split has occurred. DFSMShsm and
DFSMSdss check the ICF catalog at the start and end of a backup. If a split is in
progress at the start of a backup, the backup is not taken. If a split has occurred
during a backup, or a split is still in progress at the end of a backup, the backup is
discarded.
So, to take a BWO backup successfully, the normal time between splits must be
greater than the time taken for DFSMShsm and DFSMSdss to take a backup of the
data set.
Data tables: You can use BWO with CICS-maintained data table base clusters.
However, you cannot use BWO with user-maintained data tables, because no
forward recovery support is provided.
AIX: CICS normally uses a base key or a path key to access data in a VSAM base
cluster data set. It is also possible, but not normal, for CICS to access AIX records
by specifying the AIX name as the data set name. If an AIX data set is used in this
way, you cannot define the AIX as eligible for BWO. Instead, the AIX adopts the
BWO characteristics already defined for the VSAM sphere.
Chapter 18. Backup-while-open (BWO) 205