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CIFS access from Windows NLB

[ Edited ]

We have issue accessing network shares (located on netapp) via CIFS from Windows servers that are configured with NLB and this only happens if two or more Windows servers participating in NLB are virtual machines running of Hyper-V, at the same time no issue if NLB is configured between two phisical servers.

I found that I can fix this by turning OFF ip.fastpath

 

options ip.fastpath.enable off

 

after that all works, so questins I have:

1. if this the only way or perhaps there is something that I could adjust in NLB or Hyper-V setting to make this happen

2. what is disadventage of disabling fastpath?

from description of fastpath I know that IP fast path is a mechanism that uses the network interface of an inbound request to send the response by bypassing a routing table lookup, otherwords OFF mens more work and processing for filers. I also found someone descrabing this as follow:

"...with fastpath enabled, the filer will respond to the same MAC address and tries to send the packet back thru the same interface as it came in. This allows for loadbalancing on multiple trunk interfaces on the same subnet. Disabling this means that ONTAP has to look up its routing table to figure out where to stick the packet in and "might" introduce a delay."

 

so what is risk of disabling fastpath? what's your experiance?