I upgraded my home network to gigabit and I am very pleased with the results.  I am using a non-parity array right now, and my writes went from ~8MBps to ~32MBps.  I am happy with these numbers, but I was wondering if jumbo frames could improve it even more.  What can I say, I'm greedy!

 

Right now, here is my setup:

 

NetGear GS108 switch, supports up to 9K JF (its unmanaged, so there's no config--it's just suppose to "work"...)

unRAID (v4.3 beta6), NetGear GA311 (Realteak 8169), supports 7500 JF

HTPC, NetGear GA311 (Realtek 8169), supports 7500 JF

Den PC, Marvel Yukon gigabit, supports 1500, 4500, and 9K (it's a drop down box--does not have a "7500" option)

 

I also have 2 older PCs using integrated 10/100 NICs, and I would assume they don't even support jumbo frames.  However, I don't really care about improving performance on those machines--just maintaining connectivity.

 

So assuming I can enable jumbo frames on unRAID, my follow-on question is: Will using jumbo frames on the unRAID server and 2 main PCs cause problems talking to PCs that aren't using it?  In other words, are jumbo frames an all or nothing proposition?

 

Every machine on the same network must be able to handle the same frame size.  So no, it won't work.

 

The only way to make it work without retrofitting all of your equipment with gigabit nics (which may not be feasible) is to get a switch that supports vlans.  Basically your gigabit equipment is connected to one vlan, and everything else to another vlan (any gigabit equipment that does NOT support jumbo frames, must be lumped in the "everything else" category).  You also need to find out the highest jumbo frame all of your gigabit equipment handles, then use the lowest one (so if you have five that handle 9K but one that can only handle 7K, you need to set it to 7K).

OK, but how do you actually set the jumbo frames in unraid?

Even if they weren't a total BITCH to get and working, Jumbo frames are not going to get you squat on a local, unrouted subnet.

 

Jumbo frames are a boon to WANs and routed subnets because the reduce interrupts and thus reduce CPU.... i.e. save you money by not having to upgrade your router horsepower.

 

They improve throughput for a given rate of packet loss.... if you already have zero packet loss (typical on a local LAN) then a 9K MTU won't gain you anything perceptible.

 

TCP is a windowing protocol, and uses a sliding window up to 64K (much larger than jumbo frames) that will already take advantage of your zero packet loss environment.

 

Please see this post for an explanation of why you don't get what you expect out of Gigabit Ethernet.

 

  http://lime-technology.com/forum/index.php?topic=787.msg21068#msg21068

 

If you are pounding packets cross-country from LA to NY, jumbo frames can help as your latency is going to be at least 40 msec.  On your local subnet, your latency is more like 2ms.  None of you are trying to pass a billion packets per second and need big-bucks CPUs in your routers to do it.... so the CPU savings are meaningless.