Why shouldn't one use NMP/round robin MPIO with MSCS in ESXi < 5.5?

[rayvd]

Could someone out there explain to me why MSCS doesn't work well with round robin NMP on ESXi < v5.5?  Best explanation I've seen is that each path is "seen" differently by the guest OS and so it must continually send SCSI reset and reservation commands slowing everything down.  Presumably this could complicate recovery situations as well....

Looking for a better explanation though -- which would also help me understand how it "works" in ESXi 5.5+ now.

[chriswahl · Accepted solution]

As per Cormac Hogan:

In the past, round robin path policy could not be supported on shared storage in MSCS. This was due to the inability to release SCSI reservations on certain paths when the reservation was placed on a different path to the same disk. With the release of vSphere 5.5, this issue has now been addressed and we can support RR on storage that is being consumed by MSCS nodes.

source

[rayvd]

Thanks, Chris.  Very helpful.