Global Mirror

Global Mirror processing provides a long-distance remote copy solution across two sites for open systems or IBM® Z (or both) data by using asynchronous technology. This processing is accomplished by using the DS command-line interface (DS CLI).

Note: As of release 9.4.0, the Multi-Target Global Mirror feature is enabled, enhancing traditional Global Mirror (GM) relationships by allowing a single volume to engage in two distinct GM relationships simultaneously. For more information, refer to Multi-Target Global Mirror.

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 system over greater distances without affecting overall performance. It is also designed to provide application consistent data at a recover (or remote) site in 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 systems. 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.
Note: PowerHA® support is now included for Global Mirror on the DS8000® series. (PowerHA is the IBM Power Systems offering for high availability and disaster recovery.)

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 system, 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 benefits:
  • 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 that is 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 systems 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 system controls the creation of consistency groups in the Global Mirror session. The master storage system sends commands to subordinate storage systems. A storage system is allowed to be a master for only one Global Mirror session.
Subordinate
The subordinate storage system receives commands from a master storage system. The subordinate storage system is identified when a Global Mirror session is started. The subordinate storage system forms consistency groups and performs other Global Mirror processing. A subordinate storage system can be controlled only by one storage system master.
Session
A session is a collection of volumes across multiple storage systems 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 participate in the Global Mirror consistency group. A session is opened on each LSS in the enterprise that has or might have volumes that participate in the Global Mirror consistency group that is associated with the specific session ID.

Global Mirror supports up to 32 Global Mirror sessions per storage facility image.

You can use multiple Global Mirror sessions to fail over only data that is assigned to one host or application instead of forcing you to fail over all data if one host or application fails. This provides increased flexibility to control the scope of a failover operation and to assign different options and attributes to each session.

Control path
The control path is established from the master storage system to the subordinate storage system when more than one storage system participates in the Global Mirror session. If there is only one storage system that is involved, you do not have to create a control path. The master storage system communicates directly with its subordinate storage system.

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.
  1. Consistency groups of volumes are created at the local site.
  2. Increments of consistent data are sent to the remote site.
  3. FlashCopy operations are performed at the remote site.
  4. Global Copy operations are resumed between the local and remote site to copy out of sync tracks.
  5. The steps are repeated according to the defined time intervals.