[Wylug-help] The recent Xen talk - a question...

[James Holden]

On Thu, Sep 20, 2007 at 08:11:46PM +0100, Chris Myers wrote:
> Hi Guys,

Hi Chris,

> Something that was mentioned in the recent Xen talk.
> 
> He mentioned that he had a number of Xen servers using the same IP 
> address & that Windows machine were not capable of this. Assuming that I 
> heard correctly, I have a few questions:

Yes... fire away.

> 1. This doesn't appear to be possible. How is this achieved?

By having only one of them respond to ARP requests at a time, and a
mechanism to ensure that the privilege is passed over to another machine
in the event of a failure.

The machine wearing the hat then distributes requests off to other
machines.

IP failover is usually achieved with 'heartbeat'.
Load balancing is done with LVS.
Back-end service monitoring is often done with ldirectord.

> 2. There must be a benefit. What advantage is there for having multiple 
> machines on one IP address?

Load balancing and redundancy mainly. Our web stuff at work does this -
boxes can fail and stuff keeps ticking along.

> 3. Why can't Windows do this?

Good question ;-) Ask Bill.

> I'd love to know. :-)

It's quite hard to get your head around how it works if you've not seen
it before. Judging by most of the MCSEs I've met, it'd make their heads
explode way before they grasped it (/ducks).

Also.... there are hardware solutions to failover and load balancing
(that, in my experience, aren't very good). I suspect Windows shops are
more likely to purchase this sort of thing instead due to the shiny
factor, presumed ease of use and commercial support, as well as PHB
management appeal.

james

> 
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