Xen PV vs. KVM: The Battle for the Linux Kernel
If you are still running your critical infrastructure on OpenVZ 'containers,' you are likely sharing your kernel with a dozen other noisy neighbors. I’ve seen it happen too often: a client’s MySQL process hangs not because of a bad query, but because another user on the host node decided to compile a kernel, starving the CPU scheduler. It is time to stop accepting 'burstable' RAM as a feature. It is a liability.
With the recent release of Red Hat Enterprise Linux 6 just a few months ago, the industry standard has shifted. Red Hat has deprecated Xen in favor of KVM (Kernel-based Virtual Machine). This isn't just a vendor preference; it is a fundamental architectural change in how we handle instruction sets and memory management.
The Hypervisor Tax: Why Architecture Matters
To understand why we at CoolVDS are migrating aggressively to KVM, you have to look at the instruction path. Xen, particularly in Paravirtualization (PV) mode, relies on a modified kernel. It sits under the operating system as a bare-metal hypervisor. It’s robust, sure. Amazon builds clouds on it. But for the average high-performance VPS, it introduces overhead in the Dom0 (management domain).
KVM is different. It turns the Linux kernel itself into the hypervisor. There is no middleman translation layer for the CPU instructions—the kernel simply handles the virtual machine as a regular process.
Pro Tip: If you are migrating a legacy CentOS 5 box to a KVM host, make sure you install the `virtio` drivers. Without `virtio_net` and `virtio_blk`, you are emulating legacy hardware (like an RTL8139 network card), which will cap your throughput regardless of the host's pipe size.
War Story: The Magento Lockup
Last week, we debugged a Magento deployment for a client in Oslo. They were hosting on a legacy Xen provider and experiencing 400ms waits on disk I/O. The physical disks were SAS 15k, but the throughput was abysmal. Why? The hypervisor’s I/O scheduler was fighting with the guest OS scheduler.
We moved them to a CoolVDS KVM instance backed by enterprise SSDs. We set the guest I/O scheduler to `noop` since the hypervisor handles the sorting. The result? Wait times dropped to under 12ms.
# Check your current scheduler (CentOS 5/6)
cat /sys/block/vda/queue/scheduler
[noop] anticipatory deadline cfq
The Storage Revolution: SSDs and I/O
In 2011, spinning rust is the bottleneck. While SAS drives in RAID-10 are standard, the emergence of reliable Solid State Drives (SSDs) in the enterprise space is changing the game. However, virtualization adds latency.
Because KVM is part of the mainline Linux kernel (since 2.6.20), it benefits immediately from every improvement in the Linux scheduler. Xen requires patches and backports. When we deploy high-speed storage, KVM passes that raw IOPS performance through to the guest much more efficiently than a paravirtualized Xen kernel can.
The Norwegian Context: Latency and Law
Performance isn't just about CPU cycles; it's about network topology. If your customers are in Norway, hosting in Germany or the US adds unavoidable milliseconds. A packet from Oslo to Texas and back takes ~130ms via fiber. Locally, via NIX (Norwegian Internet Exchange), it’s <10ms.
Furthermore, we must address the Personal Data Act (Personopplysningsloven). While US Safe Harbor frameworks exist, Datatilsynet (The Norwegian Data Protection Authority) is becoming increasingly strict about data control. Running your own KVM instance gives you full block-level encryption capabilities that shared hosting or OpenVZ containers often cannot provide securely. You control the kernel; you control the encryption keys.
Why We Chose KVM for CoolVDS
We didn't just pick KVM because Red Hat did. We picked it because it allows us to offer true hardware virtualization. When you buy a VPS from us, you aren't fighting for kernel threads. You get a dedicated segment of RAM and CPU execution time.
Combined with our local presence and low latency to the Nordic backbone, this architecture ensures that your heavy PHP applications or Java heaps don't stall when the host gets busy. Don't let legacy virtualization taxes eat your margins.
Ready to test the difference? Deploy a KVM instance on CoolVDS today and check your `iowait` stats. You will see the difference immediately.