Designing Disaster Tolerant High Availability Clusters, 10th Edition, March 2003 (B7660-90013)

Building a Metropolitan Cluster Using MetroCluster/CA
Preparing an MC/ServiceGuard Cluster for MetroCluster /CA
Chapter 392
Consistency Group
An important property of asynchronous mode volumes is the
consistency group (CT group). A CT group is a grouping of LUNs that
need to be treated the same from the perspective of data consistency (I/O
ordering). A CT group is equal to a device group in the Raid Manager
configuration file. A consistency group ID (CTGID) is assigned
automatically during pair creation.
NOTE Different XP model has a different maximum number of Consistency
Groups. For details check the XP user’s guide.
Limitations of Asynchronous Mode
The following are restrictions for an asynchronous CT group in a Raid
Manager configuration file:
• Asynchronous device groups cannot be defined to extend across
multiple XP Series disk arrays.
• When making paired volumes, the Raid Manager registers a CTGID
to the XP Series disk array automatically at paircreate time, and
the device group in the configuration file is mapped to a CTGID.
Efforts to create a CTGID with a higher number will be terminated
with a return value of EX_ENOCTG.
• MetroCluster/CA will support only one consistency group per
package. This means that in one metropolitan cluster, the number of
packages can be configured to use consistency group is either limited
by the maximum number of consistency group that is supported by
the XP model in the configuration or by the maximum number of
packages in the cluster which ever is the smaller.
Other Considerations on Asynchronous Mode
The following are some additional considerations when using
asynchronous mode:
• When adding a new volume to an existing device group, the new
volume state is SMPL. The XP disk array controller (DKC) is smart
enough to do the paircreate only on the new volume. If the device
group has mixed volume states like PAIR and SMPL, the
pairvolchk returns EX_ENQVOL, and horctakeover will fail.