On Sun, Nov 22, 2009 at 08:28:24PM -0000, Matthew Melbourne wrote: > What is the general recommendation regarding enabling flow control on > Ethernet interfaces. Is it a legacy issue when devices had smaller buffers, > or is it still required for specific applications? We are having issues with > an Enterprise NAS solution where servers using it for storage are claiming > to be losing connectivity. The NAS is connected to the switch fabric (a pair > of Catalyst 6509s) by two 2*10GE port-channels (10GBase-SR optics); receive > flow control is enabled on the switch side "flowcontrol receive on", but no > input or output pause frames are being received/sent according to the member > interface statistics. > > The vendor is now suggesting that flowcontrol needs to be enabled end-to-end > - e.g. on aggregation switches downstream from the Catalyst 6509s towards > the servers and on the hosts. However, the utilisation on the NAS > port-channels is only ~400Mbps. Does enabling flowcontrol make sense here?
Storage vendors seem to blame a plethora of issues on disabled Ethernet Flow Control. Every discussion that I've ever had with any of them, every document that I've ever read, totally fails to understand what ethernet flow control does and how it works. No one is even aware of the head of line blocking problem. Remember - when you pause your NAS, you pause it for EVERYONE. Maybe I've talked to the wrong folks, but no one seems to understand this. It's almost like EMC thinks they designed their NAS for a single client... The answer is very simple: if someone thinks that ethernet flow control is the answer, the burden of proof is on them to answer difficult questions about what the actual problem is, what flow control is going to solve, and why they think that it won't cause more problems than its worth. At best it does nothing, realistically it interferes with TCP flow control, and at worst it pauses your storage and breaks every client. -- Ross Vandegrift r...@kallisti.us "If the fight gets hot, the songs get hotter. If the going gets tough, the songs get tougher." --Woody Guthrie
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