Why Xen Still Beats OpenVZ for Serious Workloads

It is 3:00 AM. Your monitoring system is screaming. The load average on your database server just spiked to 50.0. You log in, run top, and see... nothing. Your processes are idle. Yet, the SSH session is lagging.

Welcome to the "noisy neighbor" effect, the hallmark of oversold OpenVZ containers. In the world of cheap hosting, resources are often a mirage. But here at CoolVDS, we don't play probability games with your infrastructure. We bet the farm on Xen.

As of mid-2012, the virtualization landscape is split. You have the container camps (OpenVZ, LXC) and the hypervisor camps (Xen, KVM, VMware). For a developer targeting the Norwegian market, where stability and data integrity (thanks to Datatilsynet's strict enforcement of Personopplysningsloven) are paramount, understanding the architecture under your hood is not optional. It is survival.

The Architecture: Dom0, DomU, and Ring -1

Xen isn't just a fancy process manager; it's a bare-metal hypervisor. It boots before the operating system. The first VM, Domain-0 (Dom0), is the privileged administrator. Every customer VPS is a DomU (Unprivileged Domain).

Why does this matter for your MySQL performance? Because unlike containers, which share a single kernel version, Xen allows true resource isolation. If Neighbor A crashes their kernel, your DomU keeps humming along. Memory is hard-reserved, not "burstable" phantom RAM that vanishes when you need it most.

Paravirtualization (PV) vs. Hardware Virtual Machine (HVM)

In 2012, we are seeing a shift, but the distinction remains critical for performance tuning.

• Xen PV: The guest OS knows it is virtualized. It makes efficient hypercalls directly to the hypervisor. This avoids the overhead of emulating hardware. Ideally suited for Linux-to-Linux hosting.
• Xen HVM: Uses CPU virtualization extensions (Intel VT-x or AMD-V) to run unmodified operating systems (like Windows or BSD).

For a standard CentOS 6 or the new Ubuntu 12.04 LTS web server, PV is often the performance king. However, HVM with PV drivers is catching up fast.

Configuration: Controlling the Beast

If you are managing your own Xen nodes or simply want to understand what we do at CoolVDS to ensure your latency to NIX (Norwegian Internet Exchange) stays low, look at the config files.

A typical Xen configuration file in /etc/xen/ looks like this:

# /etc/xen/web-node-01.cfg
name = "web-node-01"
memory = 2048
vcpus = 2
vif = [ 'bridge=xenbr0' ]
disk = [ 'phy:/dev/vg0/web-node-01-disk,xvda,w' ]
bootloader = "/usr/bin/pygrub"

Note the disk parameter. We use phy: which maps a physical block device (usually an LVM volume) directly to the VM. Many budget providers use file-backed storage (file:/var/lib/xen/images/vm.img), which adds a massive layer of filesystem overhead. Direct LVM mapping is one reason our I/O throughput doesn't choke during backups.

The "Steal Time" Metric

How do you know if your host is oversubscribing CPUs? Look at %st (steal time) in top.

Cpu(s): 12.5%us,  2.0%sy,  0.0%ni, 80.0%id,  0.3%wa,  0.0%hi,  0.1%si,  5.1%st

If that last number, 5.1%st, consistently goes above 10-15%, your hypervisor is starving you. It means your VM wants to run, but the physical CPU is busy serving someone else. At CoolVDS, we monitor this aggressively. If a node gets too hot, we migrate VMs immediately using live migration.

Optimizing I/O for the SSD Era

We are deploying more RAID-10 SSD arrays to keep up with database demands. If you are lucky enough to be on an SSD-backed instance, the default Linux I/O scheduler (CFQ - Completely Fair Queuing) is actually a bottleneck. It anticipates rotating platters.

Pro Tip: Switch your scheduler to deadline or noop inside your guest VM for lower latency.

# Check current scheduler
cat /sys/block/xvda/queue/scheduler
[cfq] deadline noop

# Change to noop on the fly
echo noop > /sys/block/xvda/queue/scheduler

# Make it permanent in /boot/grub/menu.lst (CentOS 6)
# kernel /vmlinuz-2.6.32... elevator=noop

Privacy and the "Datatilsynet" Factor

Hosting outside of Norway means your data is subject to foreign jurisdictions. With the implementation of the EU Data Protection Directive and Norway's strict Personopplysningsloven, keeping customer data within national borders is often a legal requirement, not just a preference.

Latency matters, but sovereignty matters more. By hosting in our Oslo data center, you ensure that your data falls under Norwegian jurisdiction, satisfying local compliance officers and keeping your response times to Oslo users under 5ms.

Systems Architect Note: Never rely on default `sysctl.conf` settings for high-traffic Xen guests. Increase your `net.ipv4.tcp_max_syn_backlog` to handle connection spikes without dropping packets.

Why CoolVDS?

We don't oversell. It is that simple. When you buy 2GB of RAM on CoolVDS, that memory is reserved in the Xen hypervisor for your DomU. We don't use memory ballooning to fake capacity. We use high-performance SSD storage in RAID configurations to ensure that when you write to disk, it stays written.

In an era where everyone is racing to the bottom on price by cramming 500 containers onto a single server, we are sticking to the architectural purity of Xen. It costs us more to run, but it saves you the 3:00 AM pager alert.

Ready to stop fighting for CPU cycles? Deploy a true Xen VPS with CoolVDS today and experience the stability of dedicated resources.