vSphere Storage - ESXi 5.1 - Documentation - VMware
vSphere Storage - ESXi 5.1 - Documentation - VMware
vSphere Storage - ESXi 5.1 - Documentation - VMware
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Understanding Multipathing and 20<br />
Failover<br />
To maintain a constant connection between a host and its storage, <strong>ESXi</strong> supports multipathing. Multipathing<br />
is a technique that lets you use more than one physical path that transfers data between the host and an external<br />
storage device.<br />
In case of a failure of any element in the SAN network, such as an adapter, switch, or cable, <strong>ESXi</strong> can switch<br />
to another physical path, which does not use the failed component. This process of path switching to avoid<br />
failed components is known as path failover.<br />
In addition to path failover, multipathing provides load balancing. Load balancing is the process of distributing<br />
I/O loads across multiple physical paths. Load balancing reduces or removes potential bottlenecks.<br />
NOTE Virtual machine I/O might be delayed for up to sixty seconds while path failover takes place. These<br />
delays allow the SAN to stabilize its configuration after topology changes. In general, the I/O delays might be<br />
longer on active-passive arrays and shorter on active-active arrays.<br />
This chapter includes the following topics:<br />
n “Failover with Fibre Channel,” on page 193<br />
n “Host-Based Failover with iSCSI,” on page 194<br />
n “Array-Based Failover with iSCSI,” on page 196<br />
n “Path Failover and Virtual Machines,” on page 197<br />
n “Managing Multiple Paths,” on page 198<br />
n “<strong>VMware</strong> Multipathing Module,” on page 199<br />
n “Path Scanning and Claiming,” on page 201<br />
n “Managing <strong>Storage</strong> Paths and Multipathing Plug-Ins,” on page 205<br />
Failover with Fibre Channel<br />
To support multipathing, your host typically has two or more HBAs available. This configuration supplements<br />
the SAN multipathing configuration that generally provides one or more switches in the SAN fabric and one<br />
or more storage processors on the storage array device itself.<br />
In the following illustration, multiple physical paths connect each server with the storage device. For example,<br />
if HBA1 or the link between HBA1 and the FC switch fails, HBA2 takes over and provides the connection<br />
between the server and the switch. The process of one HBA taking over for another is called HBA failover.<br />
<strong>VMware</strong>, Inc. 193