System information

After the NMP determines which SATP to call for a specific storage device and associates the SATP with the
physical paths for that storage device, the SATP implements the tasks that include the following:
n
Monitors health of each physical path.
n
Reports changes in the state of each physical path.
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 Plugins (PSPs) run in conjunction with the VMware NMP and are responsible for choosing a
physical path for I/O requests.
The VMware NMP assigns a default PSP for every logical device based on the SATP associated with the physical
paths for that device. You can override the default PSP.
By default, the VMware NMP supports the following PSPs:
Most Recently Used
(MRU)
Selects the path the ESX/ESXi host used most recently to access the given device.
If this path becomes unavailable, the host switches to an alternative path and
continues to use the new path while it is available.
Fixed
Uses the designated preferred path, if it has been configured. Otherwise, it uses
the first working path discovered at system boot time. If the host cannot use
the preferred path, it selects a random alternative available path. The host
automatically reverts back to the preferred path as soon as that path becomes
available.
NOTE With active-passive arrays that have a Fixed path policy, path thrashing
might be a problem.
Round Robin (RR)
Uses a path selection algorithm that rotates through all available paths enabling
load balancing across the paths.
VMware NMP Flow of I/O
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 If the I/O operation is successful, the NMP reports its completion.
4 If the I/O operation reports an error, the NMP calls an appropriate SATP.
5 The SATP interprets the I/O command errors and, when appropriate, activates inactive paths.
6 The PSP is called to select a new path on which to issue the I/O.
Chapter 2 Using ESX/ESXi with Fibre Channel SAN
VMware, Inc. 25