Three Data Center Architectures

In three data center solutions, there must be an equal number of nodes (1-7) in each primary data center, and the third data center (known as the arbitrator data center) can contain 1 or 2 nodes. Cluster lock disks must not be configured.

The abritrator nodes are standard MC/ServiceGuard nodes configured in the cluster; however, they are not allowed to be connected to the shared disks in either of the primary data centers. Arbitrator nodes are used as tie-breakers to maintain cluster quorum when all communication between the two primary data centers is lost. The data center containing the arbitrator nodes must be located separately from the primary data centers.

It is possible to use a single MC/ServiceGuard Quorum Server node in place of Arbitrator node(s); however, the quorum server system must still be located in a third data center separate from the primary data centers. For more information about quorum server, refer to the manual Managing MC/ServiceGuard and to the MC/ServiceGuard Quorum Server Release Notes.

If ServiceGuard OPS Edition is used, then there can only be two nodes configured to share OPS data, as MirrorDisk/UX only supports concurrent volume group activation for up to two nodes.

There can be separate networking and Fibre Channel links between the data centers, or both networking and Fibre Channel can go over DWDM links between the data centers. See below, "Network and Data Replication Between the Data Centers" for more detail.

Fibre Channel Direct Fabric Attach (DFA) is recommended over Fibre Channel Arbitrated loop configurations, due to the superior performance of DFA, especially as the distance increases. Therefore Fibre Channel switches are preferred over Fibre Channel hubs.

Fibre Channel Arbitrated loops are limited to a maximum of four nodes and nine Fibre Channel Disk Arrays per loop. This restriction does not exist if Direct Fabric Attach configurations are used.

Any combination of the following Fibre Channel capable disk arrays may be used: Model FC30, HP Surestore FC10, HP Surestore FC60, HP SurestoreVirtual Arrays, HP Surestore Disk Array XP or EMC Symmetrix Disk Arrays.

Application data must be mirrored between the primary data centers. If MirrorDisk/UX is used, Mirror Write Cache (MWC) must be the Consistency Recovery policy defined for all mirrored logical volumes. This will allow for resynchronization of stale extents after a failure of a mirror copy, rather than requiring a full resynchronization. You must ensure that the mirror copies reside in different data centers, so it is recommended to configure physical volume groups for the disk devices in each data center, and to use Group Allocation Policy for all mirrored logical volumes.

Due to the maximum of 3 disk images (one original and two mirror copies) allowed in MirrorDisk/UX, if JBODs are used for application data, only one data center can contain JBODs while the other data center must contain disk arrays with hardware mirroring.

VERITAS Volume Manager (VxVM) mirroring is supported for clusters of 2 or 4 nodes. However, the cluster lock devices must still be configured in LVM Volume Groups. The maximum distance supported for VxVM is 10 kilometers. You must ensure that the mirror copies reside in different data centers, and the DRL (Dirty Region Logging) feature must be used. Raid 5 mirrors are not supported.

VERITAS Cluster Volume Manager (CVM) is not supported.