vSphere Storage - ESXi 5.1 - Documentation - VMware

vSphere Storage
n Performs array-specific actions necessary for storage fail-over. For example, for active-passive devices, it
can activate passive paths.
VMware PSPs
Path Selection Plug-Ins (PSPs) are sub plug-ins of the VMware NMP and are responsible for choosing a physical
path for I/O requests.
The VMware NMP assigns a default PSP for each logical device based on the SATP associated with the physical
paths for that device. You can override the default PSP. For information, see “Path Scanning and Claiming,”
on page 201.
By default, the VMware NMP supports the following PSPs:
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 most
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 at http://kb.vmware.com/kb/2003468.
The policy is displayed in the client as the Most Recently Used (VMware) path
selection policy.
VMW_PSP_FIXED The host uses the designated preferred path, if it has been configured.
Otherwise, it selects the first working path discovered at system boot time. If
you want the host to use a particular preferred path, specify it manually. Fixed
is the default policy for most active-active storage devices.
NOTE 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.
Displayed in the client as the Fixed (VMware) path selection policy.
VMW_PSP_RR The host uses an automatic path selection algorithm rotating through all active
paths when connecting to active-passive arrays, or through all available paths
when connecting to active-active arrays. 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.
VMware NMP Flow of I/O
Displayed in the client as the Round Robin (VMware) path selection policy.
When a virtual machine issues an I/O request to a storage device managed by the NMP, the following process
takes place.
1 The NMP calls the PSP assigned to this storage device.
2 The PSP selects an appropriate physical path on which to issue the I/O.
3 The NMP issues the I/O request on the path selected by the PSP.
4 If the I/O operation is successful, the NMP reports its completion.
5 If the I/O operation reports an error, the NMP calls the appropriate SATP.
200 VMware, Inc.