A path to a LUN includes the Host Bus Adapter (HBA) or initiator, the fabric/network switches, and the storage controllers in the array. The availability of a path will be affected if any of these hardware components along the path fail or stop functioning. Multipathing is a method to configure and maintain multiple paths between the host and the storage array. Although redundant fabric switches will be used to achieve this, the multipathing information available at ESXi will not show the switches involved.

Storage multipathing on an ESXi host is achieved with the help of a framework of APIs called Pluggable Storage Architecture (PSA). The APIs can be used by the storage vendors to write their own Multipathing Plugins (MPP), thus enabling a closer integration of their storage devices. Some examples for available third-party MPPs are as follows:

• EMC PowerPath.
• Dell EqualLogic MEM for iSCSI multipathing.
• VERITAS dynamic multipathing for VMware.
• The default multipathing plugin on an ESXi host is called Native Multipathing Plugin (NMP). The NMP adds support for all the supported storage arrays in the VMware compatibility list.

The NMP has two sub plugins known as Storage Array Type Plugin (SATP) and Path Selection Plugin (PSP). VMware includes the SATP and PSP associations for all tested and supported storage arrays in the form of claim rules.

SATP detects the path state and handles path failover, whereas PSP determines which available physical path should be used to send the I/O.

VMware supports the following path selection plugins:

• Most Recently Used (MRU): In the event of a path failover, this would continue to use the path even if the original path becomes accessible again. Its initial path selection happens during the boot up of ESXi, where it selects the first-discovered path as the active path. MRU is the preferred PSP for active/passive arrays and ALUA-based arrays.
• Fixed: One of the multiple paths available is marked as the preferred path. So, in the event of a preferred path becoming accessible again, it will failback to the preferred path. This is most commonly used with active/active and ALUA-based arrays.
• Round Robin (RR): This distributes I/O to all the active paths. By default, it distributes 1,000 IOs on an active path before it sends the next 1,000 IOs down the next active path.

We discussed choosing a PSP depending on the array type, so it is important to understand different array types from the multipathing perspective. The array type is determined by the mode in which it operates. There are three such types from a multipathing perspective:

• Active/active array: This supports the simultaneous ownership of a LUN by more than one storage processor.
• Active/passive array: This supports only one storage processor owning a particular LUN. A storage processor that owns a LUN or a set of LUNs becomes the active controller for those LUNs.
• Asymmetric Logical Unit Access (ALUA) based array: Similar to an active/active array, but uses the concept of optimized and unoptimized paths. Here, an unoptimized path is a data path to the LUN via the interconnect to the other controller.