Understanding and Designing Serviceguard Disaster Tolerant Architectures Fourth Edition, December 2007 (T1906-90023)

Building an Extended Distance Cluster Using Serviceguard
Two Data Center Architecture
Chapter 2 69
• MirrorDisk/UX mirroring for LVM and VxVM mirroring are
supported for clusters of 2 or 4 nodes. However, the dual cluster lock
devices can only be configured in LVM Volume Groups.
• There can be separate networking and FibreChannel links between
the two data centers, or both networking and Fibre Channel can go
over DWDM links between the two data centers. See the section
below “Network and Data Replication Links Between the Data
Centers” for more details.
• CVM 3.5 and CVM 4.1 mirroring is supported for Serviceguard and
Extended Cluster for RAC clusters. However, the dual cluster lock
devices must still be configured in LVM Volume Groups. Since
cluster lock disks are only supported for up to 4 nodes, the cluster
can contain only 2 or 4 nodes.
• MirrorDisk/UX mirroring for Shared LVM volume groups is
supported for Extended Cluster for RAC clusters containing 2 nodes.
• FibreChannel Direct Fabric Attach (DFA) is recommended over
FibreChannel 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.
• Any combination of the following FibreChannel capable disk arrays
may be used: HP StorageWorks Virtual Arrays, HP StorageWorks
Disk Array XP, Enterprise Virtual Arrays (EVA) or EMC Symmetrix
Disk Arrays. Refer to the HP Configuration Guide (available through
your HP representative) for a list of supported FibreChannel
hardware.
• 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
node crash, rather than requiring a full resynchronization. For
SLVM (concurrently activated) volume groups, Mirror Write Cache
must not be defined as the Consistency Recovery policy for mirrored
logical volumes (that is, NOMWC must be used). This means that a
full resynchronization may be required for shared volume group
mirrors after a node crash, which can have a significant impact on
recovery time. To ensure that the mirror copies reside in different
data centers, 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.