vsphere-esxi-vcenter-server-55-storage-guide

vSphere StorageAfter the NMP determines which SATP to use for a specific storage device and associates the SATP with thephysical paths for that storage device, the SATP implements the tasks that include the following:• Monitors the health of each physical path.• Reports changes in the state of each physical path.• Performs array-specific actions necessary for storage fail-over. For example, for active-passive devices,it can activate passive paths.VMware PSPsPath Selection Plug-Ins (PSPs) are sub plug-ins of the VMware NMP and are responsible for choosing aphysical path for I/O requests.The VMware NMP assigns a default PSP for each logical device based on the SATP associated with thephysical paths for that device. You can override the default PSP. For information, see “Path Scanning andClaiming,” on page 223.By default, the VMware NMP supports the following PSPs:VMW_PSP_MRUVMW_PSP_FIXEDThe host selects the path that it used most recently. When the path becomesunavailable, the host selects an alternative path. The host does not revertback to the original path when that path becomes available again. There is nopreferred path setting with the MRU policy. MRU is the default policy formost active-passive storage devices.The VMW_PSP_MRU ranking capability allows you to assign ranks toindividual paths. To set ranks to individual paths, use the esxcli storagenmp psp generic pathconfig set command. For details, see the VMwareknowledge 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.The host uses the designated preferred path, if it has been configured.Otherwise, it selects the first working path discovered at system boot time. Ifyou 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 toDead, a new path is selected as preferred. However, if you explicitlydesignate the preferred path, it will remain preferred even when it becomesinaccessible.Displayed in the client as the Fixed (VMware) path selection policy.VMW_PSP_RRThe host uses an automatic path selection algorithm rotating through allactive paths when connecting to active-passive arrays, or through allavailable paths when connecting to active-active arrays. RR is the default fora number of arrays and can be used with both active-active and activepassivearrays to implement load balancing across paths for different LUNs.Displayed in the client as the Round Robin (VMware) path selection policy.VMware NMP Flow of I/OWhen a virtual machine issues an I/O request to a storage device managed by the NMP, the followingprocess takes place.1 The NMP calls the PSP assigned to this storage device.222 VMware, Inc.