Like a lot of kids earning their nerd stripes back in the day, I
had played around with VMWare. Remember VMWare,
that program that
would let you run FreeBSD in Windows in Linux? Those were good times.
Then there was Plex86,
which was going to do the same thing, and
be Free as well, only it never really got off the ground.
load-balancing on large doses of illicit
substances. Xen is a microkernel and supporting code that presents
itself as something similar to an x86 machine to code running on top of it.
It has no device
drivers and the like; instead it lets details like those be handled by
an operating system running on top of Xen. So, you have one primary OS
instance that controls the low-level hardware, and interfaces with Xen
for configuration purposes. Then, you can have an arbitrary number of
other OSs running on top of Xen as well, all at the same time.
Back to load-balancing: So you've got 5 instances of Linux running
on your server running Xen. One OS runs the webserver, the other the
database, etc (you're a good little sysadmin; you keep everything
seperate so the hax0rs can't hurt you). Xen lets you allocate
resources to each OS instance, so you can have the webserver OS access
256MB of RAM, or 50% of your CPU cycles, for instance. But what if
your resources are maxed out, and all of a sudden your database has to
process a nasty query? Xen will let you move that database-running OS
instance to a
different physical machine, in real-time, without affecting
the OS. There are some caveats, of course: The two machines have to be
on the same subnet, and the storage space must somehow be accessible
by both of them, but that's some crazy stuff, right there.