User's Guide

DR Groups
A data replication (DR) group is a software construct comprising one or more Vdisks in an HSV
storage system so that they:
• Replicate to the same specified destination storage array
• Fail over together
• Preserve write order within the data replication collection groups
• Share a log disk
All virtual disks used for replication must belong to a DR group, and a DR group must contain at
least one Vdisk. A DR group can be thought of as a collection of copy sets.
The replicating direction of a DR group is always from a source to a destination. By default, the
storage system on which the source Vdisk is created is called the home storage system. The home
designation denotes the preferred storage system for the source and this designation can be
changed to another storage system.
A DR group contains pointers to another DR group for replication. A DR group replicating from a
home storage system to a destination system is in the original state. When replication occurs from
a storage system that was created as the destination to the home storage system (for example,
after a failover, which is discussed later), it is in a reversed state.
DR Group Properties
Properties are defined for every DR group that is created. DR group properties are described below:
• Name: A unique name given to each DR group. HP recommends that the names of replicating
DR groups at the source and destination be the same.
• DR Mode
Source: A DR group established as an active source that replicates to a passive destination.◦
◦ Destination: A DR group established as a passive destination that receives replication
data from an active source.
• Failsafe mode: When this mode is enabled, all source Vdisks become both unreadable and
unwritable if the destination Vdisk is unreachable. This condition is known as failsafe-locked
and may require immediate intervention. When the failsafe mode is disabled and the
destination Vdisk is unreachable, normal logging occurs.
• Connected system: A pointer to the storage system where the DR group is replicated.
• Write mode:
Asynchronous mode: An I/O completion acknowledgement is sent to the host immediately
after data is written to the cache at the source controller, but before the data is delivered
◦
to the cache on the destination controller. There is no requirement to wait for the I/O
completion acknowledgement from the destination controller.
Metrocluster with Continuous Access EVA also supports the Enhanced Asynchronous
mode. This Enhanced Asynchronous mode is available with EVA4x00/EVA6x00/EVA8x00
arrays with firmware v6.000 onwards and EVA4400 arrays with firmware version
09003000 or later.
Overview of EVA and Continuous Access EVA Concepts 213