The CESTPAIR command allows you to establish a PPRC volume pair
using PPRC extended distance mode and then transition the volume pair
to synchronous mode. The command includes an OPTION parameter with
the following values:
- SYNC: This value establishes a PPRC volume
pair in synchronous mode. In this mode, the volume pair reaches DUPLEX
status after the initial copy of the volume pair is completed and
the primary updates are mirrored synchronously with the I/O in progress.
As a consequence, there is propagation overhead, manageable under
103 km, but which becomes progressively significant at very long distances.
In this case, the overhead slows down the application throughput too
much to meet reasonable production objectives.
If a volume pair
is established in extended distance mode, specifying this value causes
the volume pair to change to synchronous mode.
- XD: This value establishes a PPRC volume
pair in extended distance mode. In this mode, the volume pair does
not reach DUPLEX state after the initial copy of the volume pair is
completed. The volume remains in a PENDING.XD state. The primary volume
updates complete before they are mirrored to the secondary volume.
These track updates are recorded into a bitmap and periodically copied
to the secondary volume. As a consequence, there is no guarantee that
application dependent writes are transferred in the same sequence
that they have been applied to the primary volume. Because the contents
of the copy are uncertain, the secondary copy is described as being fuzzy, as shown in Converting from one PPRC volume state to another.
The
PPRC-XD solution provides tier 4 protection, which means it is a low
cost solution for disaster protection, but the recovery point objective
(RPO) is based on how often the tertiary FlashCopy® of the secondary volumes are created.
Most customers find that once a day consistency provides the protection
they require with this solution. Since the application system I/Os
must be quiesced or stopped until the synchronization point is achieved,
PPRC-XD cannot provide protection above the tier 4 level.
Because
mirroring in extended distance mode is done asynchronously with PPRC
volume pairs, CRIT(YES) is not valid for a volume pair established
with OPTION(XD).
For additional information about the CESTPAIR command, refer to Peer-to-Peer Remote copy command descriptions.