IBM System Storage DS3000, Programming Manual

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1. Enhanced Global Mirror Groups consisting of logical drives are created at the local site.
2.
Increments of consistent data are sent to the remote site.
3. 3. Point-in-time copy operations are performed at the remote site.
These steps are repeated according to the defined mirroring intervals.
The fundamental assumptions of the Enhanced Global Mirroring premium feature are these:
v
The data link between the primary storage subsystem and the secondary storage subsystem has
significantly lower bandwidth and higher latency than the host interface. For this reason, the mirrored
data movement must be decoupled from the primary host data requests to minimize the performance
impact to the host application. Additionally, with a restricted data link unsynchronized regions
between the primary and secondary logical drives must be tracked in sufficiently small granularity to
minimize data movement. Long distance data links to remote storage subsystems require additional
networking equipment, which can result in varying throughput performance.
v
Data on the secondary logical drive must support a site-level failover for disaster recovery. For this
reason, the data on the secondary logical drive is protected during the synchronization process so that
writes to the secondary logical drive do not render the logical drive data unusable. Additionally, many
applications require the use of more than one logical drive, each of which must be mirrored in order to
support a site-level failover. In these cases, the set of logical drives must be mirrored as a set, and the
synchronization process must coordinate the data movement and synchronization intervals to create a
consistent, usable data set on the secondary storage subsystem.
Configuring controllers for Enhanced Global Mirroring
Enhanced Global Mirroring operations are performed between controllers that have same IDs on the
primary and secondary storage subsystems; that is, controller A on the primary storage subsystem
interacts only with controller A on the secondary storage subsystem and controller B on the primary
storage subsystem interacts only with controller B on the secondary storage subsystem. Enhanced Global
Mirroring operations are not attempted between controllers that have different IDs. The network
environment does not need to provide connectivity between the controllers that have different IDs;
however, it is likely to be more cost effective to use a fabric/switch configuration that does provide such
connectivity, even though the firmware does not require it.
For Fibre Channel topology the Enhanced Global Mirroring premium feature requires that one host-side
Fibre Channel port of each controller be dedicated to mirroring operations. An additional requirement for
connectivity is that the dedicated Enhanced Global Mirroring ports must be attached to a Fibre Channel
Fabric. The dedicated port is activated by the storage system administrator. Activating the dedicated port
is a different operation than enabling the premium feature. After you activate the dedicated port you
must enable the Enhanced Global Mirroring premium feature. Enabling the Enhanced Global Mirroring
premium feature performs these actions:
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Activates the storage subsystem for Fibre Channel mirroring
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Enables the controller firmware to support your creation of mirror groups and mirror pairs
When mirror groups or mirror pairs exist, you cannot deactivate the Enhanced Global Mirroring
premium feature.
An iSCSI topology does not require a dedicated port as with Fibre Channel. The activation step is not
required when setting up Enhanced Global Mirroring over iSCSI. The controller firmware maintains a list
of remote storage subsystems that the iSCSI initiator attempts to establish sessions using all portals of the
portal group. The first portal that successfully establishes an iSCSI connection is used for all subsequent
communication with that remote storage subsystem. If communication fails, a new session is attempted
using all portals in the portal group. iSCSI ports are configured at the system level on a port-by-port
basis. For configuration messaging and data transfer, the communication between the controller uses
these global settings:
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IBM System Storage DS3000, DS4000, and DS5000: Command Line Interface and Script Commands Programming Guide