6.5

Managing Path Policies
For each storage device managed by NMP, and not PowerPath, an ESXi host uses a path selection policy. If
you have a third-party PSP installed on your host, its policy also appears on the list.
Supported Path Policies
The following path policies are supported by default.
Policy Description
VMW_PSP_FIXED
The host uses the designated preferred path, if it has been congured. Otherwise, the host selects the
rst working path discovered at system boot time. If you want the host to use a particular preferred
path, specify it through the vSphere Web Client, or by using esxcli storage nmp psp fixed
deviceconfig set. See “Changing Path Policies,” on page 55.
The default policy for active-active storage devices is VMW_PSP_FIXED.
N If the host uses a default preferred path and the path's status turns to Dead, a new path is
selected as preferred. However, if you explicitly designate the preferred path, it will remain preferred
even when it becomes inaccessible.
VMW_PSP_MRU
The host selects the path that it used most recently. When the path becomes unavailable, the host
selects an alternative path. The host does not revert back to the original path when that path becomes
available again. There is no preferred path seing with the MRU policy. MRU is the default policy for
active-passive storage devices.
The VMW_PSP_MRU ranking capability allows you to assign ranks to individual paths. To set ranks to
individual paths, use the esxcli storage nmp psp generic pathconfig set command. For
details, see the VMware knowledge base article 2003468.
VMW_PSP_RR
The host uses an automatic path selection algorithm that rotates through all active paths when
connecting to active-passive arrays, or through all available paths when connecting to active-active
arrays. Automatic path selection implements load balancing across the physical paths available to your
host. Load balancing is the process of spreading I/O requests across the paths. The goal is to optimize
throughput performance such as I/O per second, megabytes per second, or response times.
VMW_PSP_RR is the default for a number of arrays and can be used with both active-active and active-
passive arrays to implement load balancing across paths for dierent LUNs.
Path Policy Effects
The type of array and the path policy determine the behavior of the host.
Policy Active/Active Array Active/Passive Array
Most Recently
Used
Administrator action is required to fail
back after path failure.
Administrator action is required to fail back after path
failure.
Fixed VMkernel resumes using the preferred
path when connectivity is restored.
VMkernel aempts to resume by using the preferred path.
This action can cause path thrashing or failure when
another SP now owns the LUN.
Round Robin No fail back. Next path in round robin scheduling is selected.
Multipathing Considerations
You should consider a number of key points when working with multipathing.
The following considerations help you with multipathing.
n
If no SATP is assigned to the device by the claim rules, the default SATP for iSCSI or FC devices is
VMW_SATP_DEFAULT_AA. The default PSP is VMW_PSP_FIXED.
n
When the system searches the SATP rules to locate a SATP for a given device, it searches the driver
rules rst. If there is no match, the vendor/model rules are searched, and nally the transport rules are
searched. If no match occurs, NMP selects a default SATP for the device.
vSphere Command-Line Interface Concepts and Examples
54 VMware, Inc.