Because of potential data integrity exposures, use the NOCOPY option
of the XADDPAIR command only in TSO command mode, not in automated
procedures. Specify NOCOPY only when installation procedures can guarantee
that the primary and secondary volumes are equivalent at the time
that you issue the command.
The NOCOPY option specifies that the primary and secondary volumes
are identical and that it is not necessary to copy the primary volume
to the recovery system. Be careful when you specify the NOCOPY option
when any of the following are true:
- You issue the XADDPAIR command as part of automatically initiated
XRC subsystem activity, because the NOCOPY option can introduce potential
data integrity exposures during a system failure.
Example: An
application may have completed a successful write operation to a primary
volume by placing the data in the storage control’s nonvolatile storage.
Before the data mover can copy the data to the recovery system, however,
the SDM system fails. If you specify NOCOPY when you restart the SDM
system, XRC does not copy the primary volume to the secondary
volume. XRC will not synchronize them even though the two volumes
may no longer contain exactly the same data. There is a data integrity
exposure here because latent updates exist, the system data mover
has not received the updates, and there is no indication of missing
data.
- The XADDPAIR command follows a DFSMSdss full-volume restore that
included the COPYVOLID parameter. In this case, you must manually
set the secondary volume serial number to a unique ID, and vary it
online.
Note: The XADDPAIR TSO command with the NOCOPY option requires
a few seconds to complete across the sysplex. If application hosts
initiate disk write updates to the volumes before the volumes are
under XRC control, those updates will not be timestamped. (Only write
I/O operations to volumes that are part of an XRC session are timestamped.)
This temporary inconsistency has passed when an XQUERY command returns
the information that there are no volumes in seqcheck status.
The XADDPAIR command with NOCOPY is equivalent to the dual copy
function with the NOCOPY option. You must use it with care.
When you specify NOCOPY, both the primary and secondary volumes must
contain exactly the same data at the point in time when the pair is
established. To ensure that the volume contents match, perform the
following steps:
- Stop all application activity to the primary volumes.
- Use DFSMSdss to dump the full primary volumes. Specify the ALLDATA
and ALLEXCP options to dump both allocated used and allocated unused
space on the volumes. (Some applications refer to the allocated unused
space of a data set.)
- Restore the full secondary volumes.
- Ensure that no applications have made updates to the primary volumes
during the volume dump and restore process.
- Issue the XADDPAIR command with the NOCOPY option.
- Once the XRC pair is established, restart the applications to
the primary volumes. If this is part of an automated procedure, you
can check for the ANTA8101I or ANTA8004I message to detect when the
pair is under XRC control.