Managing Path
Policies
For each storage
device managed by NMP, and not PowerPath, an
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 configured. Otherwise, the host
selects the first working path discovered at system boot time. If you want the
host to use a particular preferred path, specify it through the
, or by using
esxcli
storage nmp psp fixed deviceconfig set . See
Changing Path Policies.
The default policy for
active-active storage devices is
VMW_PSP_FIXED .
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 setting 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 different
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
attempts 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. |