IBM System Storage DS3000, Programming Manual

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Chapter 7. About Enhanced Global Mirroring premium feature
When you set up Enhanced Global Mirroring, you create a one-to-one relationship between the logical
drives in a remote-mirrored pair. The remote-mirrored pair consists of a
primary logical drive on a local
storage subsystem and a secondary logical drive on a storage subsystem at another site. The
primary-secondary role is defined in an Enhanced Global Mirror Group. Mirrored relationships are
created by adding a logical drive to the primary Enhanced Global Mirror Group and adding the
corresponding logical drive to the secondary Enhanced Global Mirror Group on the remote storage
subsystem. The maximum numbers of supported Enhanced Global Mirrored Pairs are listed in the
following table.
Note: Remember, most mirror operations (such as creating relationships, synchronization settings, and
mirroring role) are managed through an Enhanced Global Mirror Group.
Maximum Number of Defined Mirrors
Controller Model Per Storage subsystem
Per Enhanced Global Mirroring
Group
DCS3700 storage subsystem 32 32
DCS3700 storage subsystem with
Performance Module Controllers 128 64
The Enhanced Global Mirroring premium feature is supported only on DCS3700 storage subsystem and
DCS3700 storage subsystem with Performance Module Controller configurations.
The primary logical drive is the logical drive that accepts host I/O activity and stores application data.
When the mirror relationship is first created, data from the primary logical drive is copied in its entirety
to the secondary logical drive. This process is known as a
full synchronization and is directed by the
controller owner of the primary logical drive. During a full synchronization, the primary logical drive
remains fully accessible for all normal I/O operations.
The controller owner of the primary logical drive initiates remote writes to the secondary logical drive to
keep the data on the two logical drives synchronized.
The secondary logical drive maintains a mirror (or copy) of the data on its associated primary logical
drive. The controller owner of the secondary logical drive receives remote writes from the controller
owner of the primary logical drive but does not accept host write requests. Hosts are able to read from
the secondary logical drive, which appears as read-only.
In the event of a disaster or a catastrophic failure at the primary site, you can perform a role reversal to
promote the secondary logical drive to a primary role. Hosts then are able to read from and write to the
newly promoted logical drive, and business operations can continue.
When write operations are performed to the primary logical drive of an Enhanced Global mirrored pair,
the modified data region of the primary logical drive is tracked. Periodically, the firmware creates a new
point-in-time image of the primary logical drive and sends the changed data regions to the secondary
logical drive. When data synchronization completes, the system uses the point-in-time images on the
secondary logical drive to ensure that the data is maintained in a consistent state during subsequent
synchronization operations to the secondary logical drive.
The automatic cycle in an active Enhanced Global Mirroring 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.
© Copyright IBM Corp. 2008, 2012
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