Do FCoE upper layers have anything analagous to a TCP_MAXSEG option? That
allows an application using TCP to ask for a smaller MSS than TCP might have
chosen otherwise.
No, FCoE does not have that. The current kernel FCoE initiator initiates its
max frame size based on the associated netdev->mtu, which normally is 1500.
So, unless the LAN mtu is changed, FCoE stack is no able to use baby jumbo
frame.
So FCoE cannot say "fcoe_mtu = min(OPTIMAL_FCOEMTU,netdev->mtu)" and send-down
frames based on that?
Would a NIC over which FCoE was running be able to be of two minds of what
the MTU happens to be? I'd think that if one user of the NIC needed/wanted
and MTU > foo one would just set the MTU large enough to include foo and be
done with it?
For a nic that supports converged traffic, i.e. LAN + FCoE, I think
it's safe to assume the nic has that capability. Setting LAN MTU large
enough will work for FCoE, but it affects everyone using netdev->mtu, so
you may see degraded LAN performance, which is certainly not good.
There have been some instances of increased MTU running causing applcations to
run afoul of Nagle and what not, but I don't think it is a general truism that
using a larger MTU degrades LAN performance.
Perhaps it is just me not getting out enough but I'm still having trouble coming
to grips with the idea of a NIC that has two (or more) ideas of a valid frame
size based on ethertype or whatnot.
At the risk of another of my "Emily Litella" moments, it sounds like the NIC HW
will have to be set to use an MTU that is the max of the desired IP and FCoE MTUs
and so you are not really talking about adding one MTU concept but two so you have:
1) the "HW MTU" used by the NIC and the guts of the driver
2) the legacy networking (eg IP etc) MTU passed up through the legacy path
3) the fcoe MTU passed up through the fcoe path
where 1 is the max of 2,3
rick jones
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