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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
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
4. Write updates to the source volume receive an immediate completion because
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.
Global Mirror provides a two-site extended distance remote copy option for disaster
recovery. With Global Mirror, the data that the host writes to the storage unit at the
production (local) site is asynchronously shadowed to the storage unit at the
recovery site. The recovery site can be located well beyond the 300 km limit that is
associated with Metro Mirror with minimal delays in response times. Separating
volumes by larger distances provides greater protection against regional disasters.
Only the capabilities of the network and channel extension technologies limit
distance.
Global Mirror is based on existing Copy Services functions: Global Copy and
FlashCopy. With Global Mirror, a consistent copy of the data is automatically
maintained on the storage unit at the recovery site. This consistency is ensured if
the order of dependent write operations to disks or disk groups is maintained. With
Global Copy operations, data consistency is guaranteed at the recovery site by
creating "freeze and run" tasks and then using FlashCopy to create point-in-time
copies at the recovery site.
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DS8000 User's Guide
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, 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.
time degradation on application write operations in extended distance mode.
the synchronization cycle is independent of the updates to the source volume.
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