July 05, 2015

Round Robin PSP

Round Robin - VMW_PSP_RR

The ESXi host uses an automatic path selection algorithm rotating through all active paths when connecting to active-passive arrays, or through all available paths when connecting to active-active arrays. On supported arrays multiple paths can be active simultaneously; otherwise, the default is to rotate between the paths.

"This is the only path selection policy that uses more than one path during a data transfer session. Data is divided into multiple paths, and the paths are alternated to send data. Even though data is sent on only one path at a time, this increases the size of the pipe and  allows more data transfer in the same period of time."

"Round Robin rotates the path selection among all available optimized paths and enables basic load balancing across the paths and fabrics."

Round Robin policy provides load balancing by cycling I/O requests through all Active paths, sending a fixed (but configurable) number of I/O requests through each one in turn.

If you want the host to use a particular preferred path, specify it through the vSphere Web Client, or by using esxcli storage nmp psp roundrobin deviceconfig set.

Policy Active/Active Array Active/Passive Array
Round Robin No fail back Next path in round robin scheduling is selected
Most Recently Used Administrator action is required to failback after path failure Administrator action is required to fail back after path failure
Fixed VMkernel resumes using the preferred path when connectivity is restored VMkernel attempts to resume by using the preferred path. This action can cause path thrashing or failure when another SP now owns the LUN

Reference:

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