IBM DS8882F Introduction And Planning Manual page 60

volumes on another LSS. (An LSS is a logical grouping of up to 256 logical
volumes for which the volumes must have the same disk format, either count key
data or fixed block.)
Remote mirror and copy is an optional feature that provides data backup and
disaster recovery.
Note: You must use Fibre Channel host adapters with remote mirror and copy
functions. To see a current list of environments, configurations, networks, and
products that support remote mirror and copy functions, click Interoperability
Matrix at the following location IBM System Storage Interoperation Center (SSIC)
website (www.ibm.com/systems/support/storage/config/ssic).
The remote mirror and copy feature provides synchronous (Metro Mirror) and
asynchronous (Global Copy) data mirroring. The main difference is that the Global
Copy feature can operate at long distances, even continental distances, with
minimal impact on applications. Distance is limited only by the network and
channel extenders technology capabilities. The maximum supported distance for
Metro Mirror is 300 km.
With Metro Mirror, application write performance depends on the available
bandwidth. Global Copy enables better use of available bandwidth capacity to
allow you to include more of your data to be protected.
The enhancement to Global Copy is Global Mirror, which uses Global Copy and
the benefits of FlashCopy to form consistency groups. (A consistency group is a set
of volumes that contain consistent and current data to provide a true data backup
at a remote site.) Global Mirror uses a master storage system (along with optional
subordinate storage systems) to internally, without external automation software,
manage data consistency across volumes by using consistency groups.
Consistency groups can also be created by using the freeze and run functions of
Metro Mirror. The freeze and run functions, when used with external automation
software, provide data consistency for multiple Metro Mirror volume pairs.
The following sections describe the remote mirror and copy functions.
Synchronous mirroring (Metro Mirror)
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DS8882F Introduction and Planning Guide
Provides real-time mirroring of logical volumes (a source and a target)
between two storage systems that can be located up to 300 km from each
other. With Metro Mirror copying, the source and target volumes can be on
the same storage system or on separate storage systems. You can locate the
storage system at another site, some distance away.
Metro Mirror is a synchronous copy feature where write operations are
completed on both copies (local and remote site) before they are considered
to be complete. Synchronous mirroring means that a storage server
constantly updates a secondary copy of a volume to match changes that
are made to a source volume.
The advantage of synchronous mirroring is that there is minimal host
impact for performing the copy. The disadvantage is that since the copy
operation is synchronous, there can be an impact to application
performance because the application I/O operation is not acknowledged as
complete until the write to the target volume is also complete. The longer
the distance between primary and secondary storage systems, the greater
this impact to application I/O, and therefore, application performance.