One of the key differences between these two virtualization platforms is the ability to oversell these to clients, leading to degradation of their services. OpenVZ works best if the host isn’t overselling its RAM resources, while XEN works best if ballooning is not enabled.
From xenproject.org, “If you boot up the system with dom0 having all the memory visible to it, and then balloon down dom0 memory every time you start up a new guest, you end up having only a small amount of the original (boot time) amount of memory available in the dom0 in the end. This means the calculated parameters are not correct anymore, and you end up wasting a lot of memory for the metadata for a memory you don’t have anymore. Also ballooning down busy dom0 might have bad side effects.”
The XEN factor
While OpenVZ is touted to be best for web hosting providers and for new VPS clients, Xen is supported in both Linux and Windows. Essentially, a XEN VPS runs in its own (isolated) kernel, loads its own modules, utilizes a fully dedicated virtualized memory, I/O and scheduler, making it more suitable for clients who need high performance in a stable environment. XEN uses SWAP to improve performance. It does this by releasing RAM to compensate for heavy loads, but this needs to be monitored as too much SWAP can create issues. As a side note, if you want to load kernel modules such as CloudLinux, we recommend XEN.
The web hosting industry is fairly evenly divided in favor of either OpenVZ or XEN. For business models built to support a wide range of Operating Systems, XEN is the way to go. The same if you don’t intend to oversell. With XEN, you pretty much know what you’re getting. The tendency to oversell OpenVZ is much greater, simply because web hosting providers use that strategy to increase profits.
The OpenVZ factor
OpenVZ are considered to be more cost-effective, utilizing a single patched Linux kernel. If it uses burstable RAM, you’re affectively borrowing from a pool of shared RAM, which can lead to a RAM shortage. In this scenario, when the RAM is fully used, the VPS will stop running processes to improve performance. If, however, you use OpenVZ with VSWAP, this outperforms burstable RAM as it acts like SWAP. It essentially acts like a disk, slowing down memory access.
If you’re researching how much RAM to purchase on a VPS plan, first determine how much RAM you need. If it’s 4GB, don’t purchase a VPS with 2GB of RAM and 2GB burstable. Get 4GB upfront, plus a set amount of burstable RAM.
Looking at OpenVZ and XEN from both sides of the aisle
- OpenVZ can be more budget friendly for consumers, but you run the risk of your provider overselling its resources.
- XEN is very stable, and while it can be oversold, it normally is not.
- OpenVZ is for Linux only while XEN supports multiple operating systems.
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