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The two-data-center architecture is based on a standard MC/ServiceGuard
configuration with half of the nodes in one data center, and the
other half in another data center. Nodes can be located in separate
data centers in the same building, or even separate buildings within
the limits of FibreChannel technology. Configurations with two data
centers have the following requirements: There must be an equal number of nodes
(1 or 2) in each data center. In order to maintain cluster quorum after the loss
of an entire data center, you must configure dual cluster lock disks
(one in each data center). Since cluster lock disks are only supported
for up to 4 nodes, the cluster can contain only 2 or 4 nodes. Therefore
a three data center cluster is a preferable solution, if dual cluster
lock disks cannot be used, or if the cluster must have more than
4 nodes. To protect against the possibility of a split cluster
inherent when using dual cluster lock, at least two (three preferred)
independent paths between the two data centers must be used for
heartbeat and cluster lock I/O. Specifically, the path from the
first data center to the cluster lock at the second data center
must be different than the path from the second data center to the
cluster lock at the first data center. Preferably, at least one
of the paths for heartbeat traffic should be different from each
of the paths for cluster lock I/O. 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 detail. 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: Model FC30, HP Surestore FC10, HP Surestore
FC60, HP SurestoreVirtual Arrays, HP Surestore Disk Array XP or
EMC Symmetrix Disk Arrays. If Model FC30 disk arrays are used as cluster
lock disks, Auto trespass must be disabled for
the entire disk array. 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.
Two Data Center FibreChannel Implementations | |
Although this architecture should work with the maximum number
of nodes allowed in an MC/ServiceGuard cluster, this cluster architecture has
been tested with a maximum of 4 nodes. Therefore, the largest configuration
currently supported is 2 nodes per data center as shown in Figure 2-1 “Two Data Centers with FibreChannel
Hubs”. This configuration can be implemented using any HP-supported FibreChannel
devices. Disks must be available from all nodes using redundant
links. Not all links are shown in Figure 2-1 “Two Data Centers with FibreChannel
Hubs”. The
two cluster lock disks should be located on separate FibreChannel
loops to guard against single point of failure. The
lock disks can also be used as data disks. They must be connected
to all nodes using redundant links (not all links are shown in Figure 2-1 “Two Data Centers with FibreChannel
Hubs”). Nodes can connect to disks in the same data center using short
wave ports, and hubs can connect between data centers using long-wave
ports. This gives you a maximum distance of 10 kilometers between
data centers, making it possible to locate data centers in different
buildings. FibreChannel Using SwitchesThe two data center architecture is also possible over longer
distances using FibreChannel switches. The following is one example
of a switched two data center configuration using FibreChannel and
FDDI networking. DWDM with Two Data CentersThe following is an example of a two data center configuration
using DWDM for both storage and networking. Advantages and Disadvantages of a
Two-Data-Center Architecture | |
The advantages of a two-data-center architecture are: Only two data centers are needed, meaning less space
and less coordination between operations staff. No arbitrator nodes are needed. All systems are connected to both copies of data,
so that if a primary disk fails but the primary system stays up,
there is greater availabilty because there is no package failover.
The disadvantages of a two-data-center architecture are: There is a slight chance of
split brain syndrome. Because there are two cluster lock disks,
you would get split brain syndrome if the following occurred simultaneously: The chances are slight, however these events happening at
the same time would result in split brain syndrome and probable
data inconsistency. Planning different physical routes for both
network and data connections or adequately protecting the physical
routes greatly reduces the possibility of split brain syndrome. Software mirroring increases CPU overhead. The cluster must be either two or four nodes with
cluster lock disks. Larger clusters are not supported due to cluster
lock requirements. Although it is a low cost solution, it does require
some additional cost: FibreChannel links are required for
both local and remote connectivity. All systems must be connected to multiple copies
of the data and to both cluster lock disks.
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