addition, in a remote mirror and copy environment, a site where production
applications run might be referred to as the production site, local site, or Site
A. A site that has mirrored data of your local site might be referred to as a
recovery site, backup site, remote site, or Site B.
Remote mirror and copy differs from FlashCopy in two essential ways. First, the
source and target volumes can be on the same storage server or on separate
storage servers that are located at some distance from each other. Second, and
more significantly, remote mirror and copy does not capture the state of the source
volume at some point in time, but rather reflects all changes that were made on the
source volume to the target volume.
The following modes of operation are available:
IBM TotalStorage Global Copy
Global Copy copies data nonsynchronously and over longer distance than
is possible with Metro Mirror. In Global Copy mode, the source volume
sends a periodic, incremental copy of updated tracks to the target volume
instead of a constant stream of updates. Because the original order of
updates is not strictly maintained, this is considered a ″fuzzy″ copy. All
updates that an attached host performs on the source volume are also
performed on the target volume, but at a later time.
IBM TotalStorage Global Mirror
Global Mirror copying provides a two-site extended distance remote copy
option for disaster recovery. This solution is based on existing Global Copy
and FlashCopy. With Global Mirror, the data that the host writes to the
storage unit at the local site is asynchronously shadowed to the storage unit
at the remote site. A consistent copy of the data is then automatically
maintained on the storage unit at the remote site.
IBM TotalStorage Metro Mirror
Metro Mirror synchronously mirrors updates that are done to source
volumes. All updates that any attached host performs on the source volume
are also performed on the target volume before operation completion is
indicated. Metro Mirror copying supports a maximum distance of 300 km
(186 mi).
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
Chapter 8. Optional feature codes for the DS8000 117
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