System information

Host-Based 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 the
one or more storage processors on the storage array device itself.
In Figure 2-2, 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.
Figure 2-2. Multipathing and Failover
ESX/ESXiESX/ESXi
SP2
storage array
SP1
switch switch
HBA2 HBA1 HBA3 HBA4
Similarly, if SP1 fails or the links between SP1 and the switches breaks, SP2 takes over and provides the
connection between the switch and the storage device. This process is called SP failover. VMware ESX/ESXi
supports HBA and SP failover with its multipathing capability.
Managing Multiple Paths
To manage storage multipathing, ESX/ESXi uses a special VMkernel layer, Pluggable Storage Architecture
(PSA). The PSA is an open modular framework that coordinates the simultaneous operation of multiple
multipathing plugins (MPPs).
The VMkernel multipathing plugin that ESX/ESXi provides by default is the VMware Native Multipathing
Plugin (NMP). The NMP is an extensible module that manages subplugins. There are two types of NMP
subplugins, Storage Array Type Plugins (SATPs), and Path Selection Plugins (PSPs). SATPs and PSPs can be
built-in and provided by VMware, or can be provided by a third party.
If more multipathing functionality is required, a third party can also provide an MPP to run in addition to, or
as a replacement for, the default NMP.
When coordinating the VMware NMP and any installed third-party MPPs, the PSA performs the following
tasks:
n
Loads and unloads multipathing plugins.
n
Hides virtual machine specifics from a particular plugin.
n
Routes I/O requests for a specific logical device to the MPP managing that device.
n
Handles I/O queuing to the logical devices.
n
Implements logical device bandwidth sharing between virtual machines.
Chapter 2 Using ESX/ESXi with Fibre Channel SAN
VMware, Inc. 23