On Mon, 14 Sep 2009 14:45:02 +1200, Nathan Ward wrote:
> Unless you do it properly, and do your QoS on routers in the middle.
This is true. But there are considerations. I became curious about
this, so I did some reading to refresh my memory.
First, keep in mind that we're talking about controlling the packets of a
remote backup. That is, the largest volume of traffic is inbound to the
network/server in question. There's limited ability to control the
traffic, therefore. That is one advantage of bwlimit: it exercise
control exactly where it does the most good.
On the other hand, that control is clearly *not* where the most
information is available.
If a router is involved, it can do egress shaping on the local side.
That's best. If a router is not involved, then the server must do
ingress filtering. I did some reading about this last night, and
assuming that what I read is not out of date or otherwise inaccurate, one
cannot attach class-full queues to the ingress filter. Given that, one
can police the inbound traffic but not prioritize it. That means that
one cannot do as I originally wrote: let the backup traffic use as much
bandwidth as available, but let anything else steal traffic from it. To
do that requires shaping at the router.
Shaping is best on the router also because it knows more. It can balance
traffic between inbound traffic to multiple servers. The receiving rsync
server can only balance amongst traffic to that server. The server knows
more than a single sending rsync, but less than the router.
As mentioned above, the router and server have limited ability to
actually control the flow of remote backup traffic. What they can do is
delay the return of TCP ACK packets. In theory, this slows the inbound
traffic. But it is not ideal, as it can also cause "excess" traffic via
retries. Since the idea is to let non-backup traffic have use of the
inbound link, this is less than ideal.
So control is most effective at the sending rsync, which suggests that
bwlimit is a good approach. But the most information is available at the
receiving router, suggesting that shaping at the router is also a good
approach.
Interesting.
- Andrew
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