Global Mirror processing
provides a long-distance remote copy solution across two sites for open systems
or z/OS® (or
both) data using asynchronous technology. This processing is accomplished
using the DS Storage Manager or the DS command-line interface (CLI).
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:
- 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.
- A consistent and restartable copy of the data at the remote site, created
with minimal impact to applications at your local site.
- 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.
- 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.
- 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.
- Session
- A session is a collection of volumes across multiple storage units that
are managed together to create consistent copies of data. The session is identified
with an ID that is unique across the enterprise. The ID identifies volumes
that will participate in the Global Mirror consistency
group. A session is opened on each LSS in the enterprise that has or might
have volumes that will participate in the Global Mirror consistency
group that is associated with the specific session ID.
- Control path
- The control path is established from the master storage unit to the subordinate
storage unit when more than one storage unit participates in the Global Mirror session. If there is only one storage unit that is involved, you do
not have to create a control path. The master storage unit communicates directly
with its subordinate storage unit.
Summary: How Global Mirror works
The
automatic cycle in an active
Global Mirror session
works as follows to maintain data at a remote site to be a point-in-time consistent
copy of data at the local site.
- Consistency groups of volumes are created at the local site.
- Increments of consistent data are sent to the remote site.
- FlashCopy operations are performed at the remote site.
- Global Copy operations
are resumed between the local and remote site to copy out of sync tracks.
- The steps are repeated according to the defined time intervals.