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Chapter 18. Backup-while-open (BWO)
The BWO facility, together with other system facilities and products, allows you to
take a backup copy of a VSAM data set while it remains open for update.
Many CICS applications depend on their data sets being open for update over a
long period of time. Normally, you cannot take a backup of the data set while the
data set is open. Thus, if a failure occurs that requires forward recovery, all
updates that have been made to the data set since it was opened must be
recovered. This means that you must keep all forward recovery logs that have been
produced since the data set was opened. A heavily used data set that has been
open for update for several days or weeks might need much forward recovery.
Using BWO, only the updates that have been made since the last backup copy was
taken need to be recovered. This could considerably reduce the amount of forward
recovery that is needed.
BWO and concurrent copy
Concurrent copy improves BWO processing by eliminating the invalidation of a
BWO dump because of updates to the data set.
The following is a comparison of various kinds of dumps that you can request:
v Normal dump. Use of the data set must be quiesced so that serialization is
obtained, the data set is dumped, and serialization is released. The data set
cannot be used for the entire time.
v Concurrent copy dump. Use of the data set must be quiesced so that
serialization is obtained, concurrent copy utilization is completed within a very
short time (compared with the actual time to dump the data set), serialization is
released, and the data set is dumped. The data set can be used after concurrent
copy initialization is complete.
v BWO dump. Serialization is attempted but is not required, and the data set is
dumped. If it is eligible for BWO, the data set is dumped without serialization
and can remain in use for the entire time, but the dump can be invalidated by
update activity to the data set.
v BWO dump using concurrent copy. Serialization is attempted but is not
required, concurrent copy initialization is completed, and the data set is
dumped. If it is eligible for BWO, the data set is dumped without serialization
and can remain in use for the entire time, and updates that occur do not cause
the dump to be invalidated.
To use concurrent copy, specify the CONCURRENT keyword when you use
DFSMShsm to dump BWO data sets.
BWO and backups
The BWO function allows backups to be taken by DFSMSdss when applications
are running in continuous operation while the data set is open for update with full
data integrity of copied data.
Continuous operation is typically 24 hours-a-day for five to seven days a week.
This is feasible only for CICS VSAM file control data sets for which CICS creates
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