I'm looking at implementing traffic control via the Linux in-kernel QoS support, with the aim of reserving a certain amount of my available bandwidth for traffic to and from a SIP PBX.

The issue is that in order for QoS to work, I have to know what the bandwidth potential is for traffic coming to (and leaving from) my Linode 540. To establish QoS, you have to put an upper cap on your total bandwidth usage of slightly less than the network will provide for, so that you can make sure you manage the send queue and not some device upstream.

Thoughts?

It's very tough to do QoS on a network like Linode's from your position in the network.

It's very, very fast, but there's huge numbers of machines creating all sorts of traffic in ways beyond your ability to predict or control. QoS is really only meaningful when the node doing the QoS's decisions on whether to send or postpone a packet will meaningfully impact the network, but there's I'm guessing thousands of individual VMs each doing their thing in this case. I don't think your node acting by itself is in a position to either create or avoid congestion.

I don't expect that I'll be able to influence other nodes on the network... what I'm trying to make sure is that (for example) a group of clients downloading a large file from my HTTP server will not result in degredation of any active VOIP calls.

So far all I have to go on are guesses about what my Linode "should" be able to do, and for the QoS to work consistently I have to be very conservative about defining "should".

Well... say you did a transfer between two Linodes at the same datacenter. These can hit hundreds of megabits. Individual connections to the outside will be a good deal slower, but eg you can have multiple 10+ megabit downloads going that don't slow each other down, because the pipe where they all hit their bottleneck is vastly larger than any of the individual connections.

Running out of other resources like disk I/O are probably bigger concerns than the network, if you're worried about a bunch of people downloading fast enough to tax your linode, setting it to 100 megabits or something would probably keep that in check.