IBM DS8000 User Manual page 142

Global Copy is a nonsynchronous mirroring function and is an alternative
mirroring approach to Metro Mirror operations. Host updates to the source volume
are not delayed by waiting for the update to be confirmed by a storage unit at
your recovery site. The source volume sends a periodic, incremental copy of
updated tracks to the target volume instead of a constant stream of updates. There
is no guarantee that dependent write operations are transferred in the same
sequence that they have been applied to the source volume. This nonsynchronous
operation results in a "fuzzy copy" at the recovery site; however, through
operational procedures, you can create a point-in-time consistent copy at your
recovery site that is suitable for data migration, backup, and disaster recovery
purposes.
To ensure that a consistent copy of the data is created, you can periodically switch
from Global Copy to Metro Mirror mode. Then, either stop application I/O or
freeze write applications to source volumes and wait for all pending updates to be
copied to the recovery site. At this point, you can create a FlashCopy operation at
your recovery site to obtain a consistent data.
The Global Copy function can operate at very long distances—well beyond the 300
km distance that is supported for Metro Mirror—and with minimal impact to
applications, with the distance limited only by the network and the channel
extended technology.
During a disaster, data can be restored only to the last known consistent increment
that was created. This means that data that is written to the production site but is
waiting to be transferred to the recovery site is lost whenever the two storage units
can no longer communicate. Be aware that the use of Global Copy functions do not
guarantee against data loss. Global Mirror functions, on the other hand, provide
recoverable copies of production data at a long distance recovery site by
continually forming consistent sets of data with no significant performance impact.
This allows a quick restart at your recovery site in case of a disaster at the
production site.
The following describes the Global Copy write sequence:
1. During a Global Copy operation, the storage unit at your production site
2. After the initial copy of tracks, the storage unit series periodically starts a
3. When this process completes, the cycle is repeated. There is little response time
4. Write updates to the source volume receive an immediate completion because
Global Mirror
Global Mirror processing provides a long-distance remote copy solution across two
sites for open systems or System z (or both) data using asynchronous technology.
This processing is accomplished using the DS Storage Manager or the DS
command-line interface (CLI).
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captures information about updates to the source and periodically sends those
updates to the target volume at the recovery site.
synchronization cycle where all updated tracks, in ascending order from the
lowest numbered track, are copied from the source volume to the target
volume. The storage unit updates the target tracks with the current information
for each track, regardless of the number of updates between the time that each
track was last copied, the current time, and the order in which the updates
occurred.
degradation on application write operations in extended distance mode.
the synchronization cycle is independent of the updates to the source volume.