Information Technology Reference
In-Depth Information
contradicts the very reason for creating failover clusters. A cluster-in-a-box coni guration still
contains a single point of failure that can result in downtime of the clustered application. If the
ESXi host hosting the two-node cluster-in-a-box coni guration fails, then both nodes are lost,
and a failover does not occur. It's a relatively simple setup to coni gure and probably best suited
for learning or testing the cluster service coni gurations. You may also i nd yourself in a situa-
tion where it's needed for planned downtime or patching.
Configuration Options for Virtual Clustering
As suggested in the fi rst part of this chapter, server clusters are deployed for high availability. In a
vSphere-based outage, high availability is not achieved by using a cluster-in-a-box confi guration,
and therefore you should avoid this confi guration for any type of critical production applications
and services.
Examining Cluster-across-Boxes Configurations
Although the cluster-in-a-box scenario is more of an experimental or education tool for clus-
tering, the cluster-across-boxes coni guration provides a solid solution for critical VMs with
stringent uptime requirements—for example, the enterprise-level servers and services like SQL
Server and Exchange Server that are heavily relied upon by the bulk of end users. The cluster-
across-boxes scenario, as the name applies, draws its high availability from the fact that the two
nodes in the cluster are managed on different ESXi hosts. In the event that one of the hosts fails,
the second node of the cluster will assume ownership of the cluster group and its resources, and
the service or application will continue responding to client requests.
The cluster-across-boxes coni guration requires that VMs have access to the same shared
storage, which must reside on a Fibre Channel, FCoE, or iSCSI storage device external to the
ESXi hosts where the VMs run. The virtual hard drives that make up the operating system vol-
ume of the cluster nodes can be a standard VMDK implementation; however, the drives used as
the shared storage must be set up as a special kind of drive called a
raw
device
mapping
(RDM).
An RDM is a feature that allows a VM to establish direct access to a LUN on a SAN device. We
also discussed RDMs briel y in Chapter 6.
A cluster-across-boxes coni guration requires a more complex setup than a cluster-in-a-box
coni guration. When clustering across boxes, all proper communication between VMs and
all proper communication from VMs and storage devices must be coni gured properly.
Figure 7.5 provides details on the setup of a two-node VM cluster-across-box coni guration
using Windows Server 2008 as the guest operating system (guest OS).
