OpenVZ vs. Xen: The Truth About Container Virtualization in 2009

If you have spent any time on WebHostingTalk or discussing server setups in the #linux IRC channels lately, you know the debate is heating up. On one side, we have the OpenVZ camp: lightweight, fast, and incredibly efficient. On the other, the Xen purists demanding hardware-level isolation. As a sysadmin who has had to recover a crashed node at 3 AM because one user decided to compile a kernel inside a container, I have strong opinions.

Most hosting providers won't tell you the nitty-gritty details of how your Virtual Private Server (VPS) is sliced from the main node. They sell you "512MB RAM" and hide the reality behind a vague Service Level Agreement. Today, we are going to look under the hood at OpenVZ, specifically for the Norwegian market where reliability and compliance with the Personal Data Act (Personopplysningsloven) are non-negotiable.

The Architecture: Shared Kernel vs. Isolation

To understand the pros and cons, you have to understand the architecture. OpenVZ uses OS-level virtualization. There is one Linux kernel running on the host node (hardware), and every VPS acts as a container (VE) sitting on top of it.

In contrast, Xen (used for our premium segments at CoolVDS) offers paravirtualization, where each VPS boots its own kernel. Why does this matter? Let's look at the trade-offs.

The Pros: Speed and Density

OpenVZ is fast. Because there is no hypervisor overhead emulating hardware or managing separate kernels, the I/O performance is nearly native. For serving static content via nginx 0.7 or running standard LAMP stacks (Linux, Apache, MySQL, PHP), it flies.

• Resource Efficiency: You aren't wasting RAM on duplicate kernel processes.
• Burst Resources: OpenVZ has a concept of "Burst RAM". If the node has free memory, your container can grab it temporarily to handle a traffic spike.
• Instant Provisioning: We can spin up a CentOS 5 template in seconds.

The Cons: The "Noisy Neighbor" and Kernel Modules

Here is where the "Battle-Hardened" reality sets in. Because you share a kernel, you are at the mercy of the node's configuration.

1. The iptables Headache:
Ever tried to run a VPN or a complex firewall rule and got a `modprobe: FATAL` error? That's because you can't load kernel modules inside an OpenVZ container unless the host admin (me) explicitly enables them for you. If you need `tun/tap` devices or specific `conntrack` modules, you better have a responsive host.

2. Resource Limits (Beancounters):
OpenVZ uses a file called `/proc/user_beancounters` to limit resources. It’s not just CPU and RAM; it limits the number of open files, socket buffers, and iptables entries.

Pro Tip: If your MySQL service is crashing randomly but you see free RAM, check the `kmemsize` or `numothersock` limits. Run this command to see if you are hitting fail counters:

cat /proc/user_beancounters | grep -v " 0$"

The Overselling Trap

The biggest criticism of OpenVZ is how easy it is for budget hosts to oversell. Since not everyone uses their RAM at once, a provider can put 50GB of RAM worth of customers on a server with only 16GB of physical RAM. Everything works fine until Friday night traffic hits, swap thrashes, and the disk I/O latency spikes through the roof.

This is critical for Norwegian businesses targeting customers in Oslo or Bergen. If your VPS is hosted on an overloaded node in a budget datacenter, your latency to the NIX (Norwegian Internet Exchange) won't matter because your disk wait time (iowait) will kill your application performance.

When to Choose OpenVZ (and CoolVDS)

Does this mean OpenVZ is bad? Absolutely not. It is a mature, stable technology if managed honestly. We use OpenVZ at CoolVDS for our standard tier because it offers the best price-to-performance ratio for web servers, DNS slaves, and development sandboxes.

The Verdict

If you need to tweak the kernel, mount FUSE file systems, or run Java heaps that require guaranteed memory allocation without fail counters, look at our Xen or emerging KVM plans. But for 90% of PHP/MySQL workloads, OpenVZ on a non-oversold node is unbeatable.

At CoolVDS, we monitor our host nodes strictly. We don't gamble with your uptime just to squeeze in one more client. Plus, our servers are located right here in the region, ensuring your data stays within the jurisdiction of the Data Inspectorate (Datatilsynet) and ping times to Oslo stay low.

Ready to upgrade your infrastructure? Stop fighting with slow I/O. Deploy a high-speed VPS Norway instance on CoolVDS today and see the difference a properly managed node makes.