I just had some thoughts cross my mind about oversold VPSes. If a hosting provider oversells their bandwidth and one or more of their clients demands more juice it just doesn't come; there is a lag and the system acts as if the internet itself is choked up. Everything waits for available bandwidth and adapts around it seamlessly. Now supposing the same host oversells CPU power and has a ton of VPSes with tiny memory allotments. If one or more clients start pounding away then their load average, and to an extent the load of the physical server will go up and all tasks will slow down a bit, very similar to an isolated physical machine that was running some intense application that was slowing everything down. Well in this case too the system will adapt to it and just run things slower. If a host oversells hard drive space nobody will know about it until space on the physical box runs out so until then it's totally transparent. But what about RAM? If a host oversells RAM and the VPS thinks a certain block of memory is there but it's really not then what? Supposing 99% of the RAM in a physical box is currently in use and suddenly the demand rises to 110% of the available RAM. Seems to me like applications on the VPS will not slow down but rather crash and vanish into thin air. Or try to start and as they load more and more RAM up suddeny they will just go poof! Taking user data with them. Am I wrong about this and has openvz impremented some failsafe mechanism to prevent this from happening? Because if not it means anyone who runs anything in an oversold(RAM) VPS is risking losing or at the very least corrupting their data.
Did you forget about SWAP? If the server uses all of its RAM, then it goes into swap... I would say by the time SWAP is filled, a sysadmin has been alerted to the resource usage. Most of the time if data is going to be dropped, its dumped into a core file if mysql is in use. Overselling ram, just isn't common - if it is being done, the moron who manages the servers needs some more bananas for his tree supply.
Yes I totally forgot about SWAP. What was I thinking? (or not thinking). But there is a preference of people for Xen VPSes over OpenVZ VPSes because the rumor is that Xen does not allow RAM to be oversold whereas OpenVZ does allow it with the whole concept of "burst ram".
For burst ram to be active, you first need to top your dedicated ram usage. So if anyone use all the burst ram, you are suppose to have at minimum your guaranteed ram available.
VMware handles it pretty easily, you can set "x" amount of ram per host, then it will grab from available resources as it gets "eaten" up. Check out vmware.com and look at their esxi product (free).
Great product. Any idea if it's better then hypervm at managing the VPS? Never really compared the two...