IBM DS8000 User Manual page 143

Global Mirror processing is most often associated with disaster recovery or
preparing for disaster recovery. However, it can also be used for everyday
processing and data migration.
The Global Mirror function is designed to mirror data between volume pairs of a
storage unit over greater distances without affecting overall performance. It is also
designed to provide application consistent data at a recover (or remote) site in case
of a disaster at the local site. By creating a set of remote volumes every few
seconds, this function addresses the consistency problem that can be created when
large databases and volumes span multiple storage units. With Global Mirror, the
data at the remote site is maintained to be a point-in-time consistent copy of the
data at the local site.
Global Mirror is based on existing Copy Services functions: Global Copy and
FlashCopy. Global Mirror operations periodically invoke a point-in-time FlashCopy
at the recovery site, at regular intervals, without disrupting the I/O to the source
volume, thus giving a continuously updating, nearly up-to-date data backup. Then,
by grouping many volumes into a session, which is managed by the master
storage unit, you can copy multiple volumes to the recovery site simultaneously
while maintaining point-in-time consistency across those volumes.
Your reasons for using Global Mirror processing might include the following:
v Support for virtually unlimited distances between the local and remote sites,
with the distance typically limited only by the capabilities of your network and
the channel extension technology. This unlimited distance enables you to choose
your remote site location based on business needs and enables site separation to
add protection from localized disasters.
v A consistent and restartable copy of the data at the remote site, created with
minimal impact to applications at your local site.
v Data currency, where your remote site might lag behind your local site by 3 to 5
seconds, minimizing the amount of data exposure in the event of an unplanned
outage. The actual lag in data currency that you experience can depend upon a
number of factors, including specific workload characteristics and bandwidth
between the local and remote sites.
v Session support whereby data consistency at the remote site is internally
managed across up to eight storage units that are located across the local and
remote sites.
v Efficient synchronization of the local and remote sites with support for failover
and failback modes, helping to reduce the time that is required to switch back to
the local site after a planned or unplanned outage.
To better understand how Global Mirror works, you must be familiar with the
following terms:
Master
The master storage unit controls the creation of consistency groups in the
Global Mirror session. The master storage unit sends commands to
subordinate storage units. A storage unit is allowed to be a master for only
one Global Mirror session.
Subordinate
The subordinate storage unit receives commands from a master storage
unit. The subordinate storage unit is identified when a Global Mirror
session is started. The subordinate storage unit forms consistency groups
and performs other Global Mirror processing. A subordinate storage unit
can only be controlled by one storage unit master.
Chapter 8. Optional feature codes for the DS8000
115