System information

Using Cluster Services
Server clustering is a method of tying two or more servers together by using a high-speed network connection
so that the group of servers functions as a single, logical server. If one of the servers fails, the other servers in
the cluster continue operating, picking up the operations that the failed server performs.
VMware tests Microsoft Cluster Service in conjunction with ESX/ESXi systems, but other cluster solutions
might also work. Different configuration options are available for achieving failover with clustering:
Cluster in a box
Two virtual machines on one host act as failover servers for each other. When
one virtual machine fails, the other takes over. This configuration does not
protect against host failures and is most commonly used during testing of the
clustered application.
Cluster across boxes
A virtual machine on an ESX/ESXi host has a matching virtual machine on
another ESX/ESXi host.
Physical to virtual
clustering (N+1
clustering)
A virtual machine on an ESX/ESXi host acts as a failover server for a physical
server. Because virtual machines that run on a single host can act as failover
servers for numerous physical servers, this clustering method is a cost-effective
N+1 solution.
Server Failover and Storage Considerations
For each type of server failover, you must consider storage issues.
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Approaches to server failover work only if each server has access to the same storage. Because multiple
servers require a lot of disk space, and because failover for the storage array complements failover for the
server, SANs are usually employed in conjunction with server failover.
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When you design a SAN to work in conjunction with server failover, all LUNs that are used by the clustered
virtual machines must be detected by all ESX/ESXi hosts. This requirement is counterintuitive for SAN
administrators, but is appropriate when using virtual machines.
Although a LUN is accessible to a host, all virtual machines on that host do not necessarily have access to
all data on that LUN. A virtual machine can access only the virtual disks for which it has been configured.
In case of a configuration error, virtual disks are locked when the virtual machine boots so that no
corruption occurs.
NOTE As a rule, when you are using boot from a SAN LUN, only the host that is booting from a LUN should
see each boot LUN. An exception is when you are trying to recover from a failure by pointing a second host
to the same LUN. In this case, the SAN LUN in question is not really booting from SAN. No host is booting
from the SAN LUN because it is corrupted. The SAN LUN is a nonboot LUN that is made visible to a host.
Optimizing Resource Use
VMware vSphere allows you to optimize resource allocation by migrating virtual machines from overused
hosts to underused hosts.
You have the following options:
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Migrate virtual machines manually by using VMotion.
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Migrate virtual machines automatically by using VMware DRS.
You can use VMotion or DRS only if the virtual disks are located on shared storage accessible to multiple
servers. In most cases, SAN storage is used.
Chapter 2 Using ESX/ESXi with Fibre Channel SAN
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