Enhanced Remote Mirroring (Erm) - IBM Midrange System DS4000 Series Hardware Manual

The VolumeCopy premium feature must be enabled by purchasing a feature key. For efficient
use of VolumeCopy, FlashCopy must be installed as well.

4.2.6 Enhanced Remote Mirroring (ERM)

The Enhanced Remote Mirroring option is a premium feature that comes with the Storage
Manager software and is enabled by purchasing a premium feature key. The Enhanced
Remote Mirroring option is used for online, real-time replication of data between two DS5000
storage subsystems over a remote distance.
The Enhanced Remote Mirroring is a redesign of the former Remote Volume Mirroring and
offers both synchronous mirroring mode or asynchronous mirroring mode. Synchronous
mode is also called Metro Mirror. Asynchronous mirroring with the consistency group option is
referred to as Global Mirror, and asynchronous mirroring without the consistency group option
is referred to as Global Copy.
Here we explain each method:
Metro Mirror
Metro Mirror is a synchronous mirroring mode. Any host write request is written to the
primary (local) storage subsystem and then copied to the secondary (remote) storage
subsystem. The host application must wait until receiving acknowledgement that the write
request has been executed on both (local and remote) storage controllers.
In the event of a communication failure, synchronous write mode offers the best chance of
full data recovery from the secondary logical drive, so it is the recommended write mode
when the maximum distance between the DS5000 storage subsystems in an Enhanced
Remote Mirroring configuration is up to 10 km, although because it performs synchronous
writes, host I/O performance is slower than it is in asynchronous write mode.
Global Copy
Global Copy is an asynchronous write mode. All write requests from a host are written to
the primary (local) storage subsystem and immediately reported as completed to the host
system. The secondary (target) is written to at a later time independently. This capability is
designed to prevent primary performance from being affected by wait time from writes on
the secondary system. Therefore, the primary and secondary copies can be separated by
long distances. This function is appropriate for remote data migration, offsite backups, and
transmission of inactive database logs at virtually unlimited distances.
Global Mirror
This mode of asynchronous copy must be used when the host application has dependent
data spread across several logical drives to ensure that the dependent write requests are
carried out in the same order at the remote site. This is done through the use of a
consistency group. Global Mirror ensures that write requests to multiple primary logical
drives are carried out in the same order on the secondary logical drives as they are on the
primary logical drives, preventing data on the secondary logical drives from becoming
inconsistent with the data on the primary logical drives
Asynchronous write mode, both Global Mirror or Global Copy, offers faster host I/O
performance than synchronous write mode, but does not guarantee that the copy has
been successfully completed before processing the next write request. This is the
recommended mode when the maximum distance between the DS4000 or DS5000
storage subsystems in an Enhanced Remote Mirroring configuration is more than 10 km.
Enhanced Remote Mirroring has also been equipped with new functions for better business
continuance solution design and maintenance tasks.
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IBM Midrange System Storage Hardware Guide