vSphere Storage - ESXi 5.1 - Documentation - VMware

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