IBM TotalStorage DS6000 Introduction And Planning Manual page 25

Global Copy
Global Copy functions offer a nonsynchronous long-distance copy option whereby
write operations to a storage unit at your production site are considered complete
before they are transmitted to a storage unit at your recovery site.
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 that is 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
captures information about updates to the source and periodically sends those
updates to the target volume at the recovery site.
2. After the initial copy of tracks, the storage unit series periodically starts a
synchronization cycle where all updated tracks, in ascending order from the
lowest numbered track, is copied from the source volume to the target volume.
The storage unit updates the target tracks with the current information for that
track, regardless of the number of updates between the time that it was last
copied, the current time, and the order in which the updates occurred.
3. When this process completes, the cycle is repeated. There is little response
time degradation on application write operations in extended distance mode.
4. Write updates to the source volume receive an immediate completion because
the synchronization cycle is independent of the updates to the source volume.
Global Mirror
Global Mirror provides a long-distance remote copy solution across two sites for
open systems or z/OS (or both) data using asynchronous technology.
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Chapter 1. Introduction