Unixshell#, the VPS hosting company that I used for this site, recently had hardware issue with one of its dom0 box, which rendered many VPS hosted on that machine offline for a few days. It seems to be a complicated hardware issue as DC has replaced almost all components to try to single out the exact cause, but at the end Matt could only bring up that node with half the memory.

One disgruntled customer reported this incident on WHT, and Matt’s reply revealed something that I considered a potential issue for Xen-based VPS hosting.

This is one huge problem we’ve run into with Xen, since memory cannot be over-committed if we run into a problem where we lose a couple of GB in a server then we have problems. With Virtuozzo/OpenVZ/Linux Vserver/FreeVPS we would be able to start up all customers at the expense of some performance.

You cannot oversell memory nor disk space on Xen — which is supposed to be a good thing for customers to ensure that the server will never be overcrowded. Each megabyte of memory in domU correspond to physical memory in dom0. If the physical memory on dom0 is less than the total amount of memory required by all domU nodes, you’ll find it is impossible start all the VPS on that machine.

Which is what happened to Unixshell in this case with VM5. Dom0 can now only start with half the memory, and they have to either (1) prevent some VPS from starting (2) reduce the memory allocation to some. Not good for the host, and not good for the customers as well, especially in the situation where no hot space is available on DC.

Where as in the case of OpenVZ/Virtuozzo, each VPS is guaranteed with a fixed amount of virtual memory, and that is physical + swap combined. In the case of this disaster, the host could still start everyone up, and increasing the swap space if they have to. Just that it will be faster to eat into the swap, which then will cause degraded performance for everyone.

Having setting up OpenVZ nodes for developers at work, I actually felt VZ is more efficient and has better resource utilisation for the host. As a customer of VPS I might prefer Xen as it feels more like a dedicated, but we also need to understand that it also runs similar risks as dedicated servers. When there is not enough physical resource, you’ll have problems.