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.
7. Note that PSP check every 300 seconds if the path is active again. SATP is responsible for doing the failover.