The round-robin algorithm rotates path selection for a volume through all paths. For any volume that uses the fixed-path policy, the first discovered preferred node path is chosen. The VMW_PSP_MRU  policy selects the first working path, discovered at system boot time. If this path becomes unavailable, the ESXi/ESX host switches to an alternative path and continues to use the new path while it is available. 

All these algorithms were modified with V4.0 and later to honor the IBM Spectrum Virtualize preferred node that is discovered by using the TPGS  command. Path failover is automatic in all cases. If the round-robin algorithm is used, path failback might not return to a preferred node path. Therefore, manually check pathing after any maintenance or problems occur. 

Update: From vSphere version 5.5 and later, VMware multipath driver fully supports IBM Spectrum Virtualize/Storwize V7000 ALUA preferred path algorithms. VMware administrators should select Round Robin and validate that VMW_SATP_ALUA is displayed . This configuration reduces operational burden and improves cache hit rate by sending the I/O to the preferred node. 

Our recommendation for NMP Round Robin configuration is to change the default value of IOs before changing a path from the default 1,000 to 1. This will better spread the workload among the paths, avoiding some high latency situations that are not visible at a SAN level. For more information about how to change this setting, see the following VMware KB : 

## 6.11.2 Multipathing configuration maximums 

https://kb.vmware.com/s/article/2069356 

The VMware multipathing software supports the following maximum configuration: 

GLYPH<SM590000> A total of 256 SCSI devices 

GLYPH<SM590000> Up to 32 paths to each volume 

GLYPH<SM590000> Up to 4096 paths per server 

Tip:  Each path to a volume equates to a single SCSI device. 

See the following VMware website for a complete list of maximums: 

https://configmax.vmware.com/# 

