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I need to subdivide EC2 (or perhaps other cloud provider) instances. I know you cannot run a full virtualization stack on another. For instance a Xen guest cannot host a VirtualBox guest. So this means I am left with operating system-level virtualization.

EC2 kernels are only publishable by select vendors, so you cannot upload the custom kernels required for OpenVZ or Vserver.

For Linux, I think this leaves me with lxc (on Ubuntu 9.10), User Mode Linux, or qemu. I'm having a hard time finding comparisons between them. Performance is a concern, as is the ability to provide SMP to the guests. I would also like to use COW/sparse roots to reduce guest provisioning.

My question is, what are the trade-offs between these options?

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I think this is a bad idea, I'm not aware of anyone even remotely in favour of routinely VMing inside a VM, it should be a test/dev thing if done at all - I suggest you think of getting another instance, running your services inside one VM or similar.

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@Chopper3, Why isn't anyone in favor of this? – recampbell Mar 31 '10 at 18:01
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You already have a time-sliced, private-memory-spaced VM with access to real resources via virtual stubs, to then sit another hypervisor on top of this and ask it to sub-divide itself, time-slice again, memory manage again, present it's devices via a secondary virtual API and manage these secondary VMs either just plain doesn't work or if it does it introduces pretty large inefficiencies/overheads and instability that most people think it's rather overegging the pudding. Can be useful for very short periods or where performance REALLY doesn't matter. – Chopper3 Mar 31 '10 at 18:18
+1 to @Chopper3's comment. @recampbell, can you give some details on why you want to do this ? – gareth_bowles Apr 1 '10 at 1:17
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Placing a VM inside a VM is a guaranteed recipe for disaster. You're in for kernel soft lockups, panics, and general unwanted nastiness. Don't Do It. – Tom O'Connor May 5 '10 at 12:48
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