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Peter Mueller <pmueller@xxxxxxxxxxxx> said:
> I definitely wouldn't use nat, even
> on 2.4. Sorry I just don't see the advantage on a site expecting 20
> million hits a day.
More important than not seeing an advantage, is that you also don't see any
disadvantage.
NAT is not -bad-. The additional latencies are mostly irrelevant. The only
real issue is the sheer number of packets being handled by the network
interfaces (as pointed out by Joe already).
All of the major commercial load balancers (Radware WSD, F5 BigIP, Alteon 180
series and Cisco LocalDirector) are NAT boxes. They work just fine. They also
all have optional gigabit interfaces. I worked at a company that was handling
30mil pages/day (~4 megabytes/sec average outbound traffic) on 10 Sun 220Rs
behind Alteons. NAT was definately -not- an issue.
As far as I'm concerned, DR and TUN are nice toys, but not very useful. They
don't offer signicant advantages over NAT (in most applications) and they
require too much special handling to manage. I can see TUN -seeming to- have
an advantage in being able to balance across different datacenters, but
that's largely offset by the single point of failure. I.e., if the datacenter
where the directors reside goes offline, all the clusters distributed to
other datacenters won't be getting any traffic anyway, unless you change the
DNS to point to another director in the other datacenter. In which case, the
F5 3DNS, which does metrics, would be a better solution than tunneling.
DR also seems to offer an advantage, but the packets are coming in through a
router and switch. The return packets may be bypassing the director, but they
are almost certainly going back out through the same switch and router. The
difference added by having the outbound packet headers re-written by the
director isn't enough to justify the added complexity of the system.
The only real reason to even use DR is if the number of packets in/out
exceeds what a gigabit connection can handle. But at what point does that
happen? I've personally seen a cluster of ~140 real-servers behind an F5
BigIP with gigabit interfaces that was handling > 20 megabytes/sec sustained
outbound traffic, most of which was small packets. I don't know how many
pages/day this was, but I can probably safely assume that it was about 5
times the system that I used to manage, so I can guess that it was somewhere
around 150 million pages/day. This was with NAT, and with a lot of special
packet handling scripts due to the complex multi-tier configuration of the
cluster, and yet the site was very fast. NAT was not an problem.
I think that the HOWTO does a good job of pointing out the advantages of DR
and TUN, but it tends to give people the impression that they are superior
and that NAT is inferior, which it really isn't. DR and TUN are handy -if you
need them-. Do you need them? Probably not.
-=dwh=-
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