Re: Integrating Replication into Core

Markus Schiltknecht wrote:

> LOL, I've just figured that netem is the project behind:
>
> tc qdisc ... netem ...
>
> I'm already using that, too ;-)  Just wasn't aware it's called netem. 
> Sounds silly, since the name is in the command line, I know...

Heh. AFAIK netem is the tc stuff that isn't much use for production 
router use (e.g.
introduce a 10ms packet delay on this kind of traffic...). We used a 
mixture of
netem and regular tc kernel modules, in a Linux box that had 6 NICs, 
with Python
driving it. Each replication node test machine was connected with a 
straight-through
patch cable to one of the NICs on the 'spider' machine. The Python could 
set up
the netem/tc on the router such that various test scenarios with 
different banwidth/delay
values were implemented. Also of course loss of connectivity by dropping 
all packets
on an interface. Each test machine had two NICs - the second one
being used to communicate with it out of band from the replication 
traffic and
network emulation.  Then on top of all this the actual replication tests 
were run.
One of the things we were interested in was replication throughput vs
network latency, so we also measured performance and made that being 
acceptable a test pass
condition.

If you want really fancy network emulation you'd need to use nistnet.
It can do some things that are not possible with netem (statistical packet
drop for example). However IMHO this is only appropriate for testing
TCP/IP stack implementation. Varying latency, throughput, and 
introducing connectivity outages
is good enough for user mode code I believe. Nistnet is not in the stock 
kernel,
wheras netem is.