6.5

Table Of Contents
2 Remove any users of the device.
You cannot bring a device back without removing active users. The ESXi host cannot know whether the
device that was added back has changed. ESXi must be able to treat the device similarly to a new device
being discovered.
3 Perform a rescan to get the device back in working order.
Managing Paths
To maintain a constant connection between an ESXi host and its storage, ESXi supports multipathing. With
multipathing you can use more than one physical path for transferring data between the ESXi host and the
external storage device.
In case of failure of an element in the SAN network, such as an HBA, switch, or cable, the ESXi host can fail
over to another physical path. On some devices, multipathing also oers load balancing, which redistributes
I/O loads between multiple paths to reduce or eliminate potential bolenecks.
The storage architecture in vSphere 4.0 and later supports a special VMkernel layer, Pluggable Storage
Architecture (PSA). The PSA is an open modular framework that coordinates the simultaneous operation of
multiple multipathing plug-ins (MPPs). You can manage PSA using ESXCLI commands. See Chapter 6,
“Managing Third-Party Storage Arrays,” on page 101. This section assumes you are using only PSA plug-ins
included in vSphere by default.
Multipathing with Local Storage and FC SANs
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 a simple multipathing local storage topology, you can use one ESXi host with two HBAs. The ESXi host
connects to a dual-port local storage system through two cables. This conguration ensures fault tolerance if
one of the connection elements between the ESXi host and the local storage system fails.
To support path switching with FC SAN, the ESXi host typically has two HBAs available from which the
storage array can be reached through one or more switches. Alternatively, the setup can include one HBA
and two storage processors so that the HBA can use a dierent path to reach the disk array.
In FC Multipathing, multiple paths connect each host with the storage device. For example, if HBA1 or the
link between HBA1 and the 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.
vSphere Command-Line Interface Concepts and Examples
50 VMware, Inc.