Enhanced Remote Mirroring; Role Reversals - IBM System Storage DS3000 Programming Manual

When you create a FlashCopy logical drive, a FlashCopy repository logical drive is automatically created.
The FlashCopy repository logical drive stores information about the data that has changed since the
FlashCopy logical drive was created. You cannot select a FlashCopy repository logical drive as a source
logical drive or a target logical drive in a VolumeCopy.
You can use the FlashCopy Logical Drive premium feature with the VolumeCopy premium feature to
back up data on the same storage subsystem and to restore the data on the FlashCopy logical drive back
to its original base logical drive.

Enhanced Remote Mirroring

The Enhanced Remote Mirroring premium feature provides for online, real-time replication of data
between storage subsystems over a remote distance. In the event of a disaster or a catastrophic failure of
one storage subsystem, you can promote a secondary storage subsystem to take over responsibility for
data storage.
When you create a remote mirror, a remote mirror pair is created, which consists of a primary logical
drive at the primary storage subsystem and a secondary logical drive at a remote storage subsystem.
The primary logical drive is the logical drive that accepts host I/O and stores data. When the mirror
relationship is initially 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 activity.
The controller owner of the primary logical drive starts remote writes to the secondary logical drive to
keep the data on the two logical drives synchronized. Whenever the data on the primary logical drive
and the secondary logical drive becomes unsynchronized, the controller owner of the primary logical
drive starts a resynchronization, where only the data that changed during the interruption is copied.
The secondary logical drive maintains a mirror 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.
The secondary logical drive stays available to host applications as read-only while mirroring is underway.
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 are then able to access the newly promoted
logical drive, and business operations can continue.
You can select a primary logical drive that is participating in a remote mirror pair to be used as the
source logical drive or a target logical drive for a VolumeCopy. A secondary logical drive that is
participating in a remote-mirror pair cannot be selected as a source logical drive or a target logical drive.

Role Reversals

A role reversal is the act of promoting the secondary logical drive to be the primary logical drive of the
remote mirror pair, and demoting the primary logical drive to be the secondary logical drive.
In the event of a disaster at the storage subsystem that contains the primary logical drive, you can fail
over to the secondary site by performing a role reversal to promote the secondary logical drive to the
primary logical drive role. This action lets hosts continue to access data, and business operations can
continue.
Trying a role reversal in which the original primary logical drive is the source logical drive for an active
VolumeCopy (the status is In Progress or Pending) causes the VolumeCopy to fail. The failure occurs
when the original primary logical drive becomes the new secondary logical drive. You cannot restart the
VolumeCopy until you return the roles of the logical drives back to their original state; that is, the logical
drive that was originally the primary logical drive is set once again to be the primary logical drive.
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IBM System Storage DS3000, DS4000, and DS5000: Command Line Interface and Script Commands Programming Guide