KVM vs. OpenVZ: The Truth About "Dedicated" Resources

Let’s be honest: the budget VPS market is a minefield of overselling. You buy a slice with "512MB RAM" and "2 Cores," deploy your LAMP stack, and then wonder why your MySQL queries hang for 3 seconds during peak hours. The culprit isn't usually your code; it's your neighbor.

In the world of 2012 hosting, the battle line is drawn between OpenVZ (Containerization) and KVM (Kernel-based Virtual Machine). As a systems architect managing infrastructure across the Nordics, I've seen too many developers seduced by the low price of OpenVZ, only to burn out debugging random latency spikes caused by resource contention.

If you are running a production workload—whether it's a high-traffic Magento store or a custom Java application—you need to understand why hardware virtualization is non-negotiable.

The Architecture: Shared Kernel vs. Private Kernel

OpenVZ is essentially a fancy chroot. All containers on the host node share the same Linux kernel. If one user discovers a kernel panic exploit, the whole node goes down. More importantly, resources like RAM and CPU are often "burstable," which is marketing speak for "you don't actually own this."

KVM, which we treat as the standard implementation at CoolVDS, turns the Linux kernel into a hypervisor. Each guest VM has its own private kernel (Linux, BSD, Windows). This means:

• True Isolation: A crash in a neighbor's kernel does not affect you.
• Dedicated RAM: No "burst" limits. If you pay for 4GB, the hypervisor reserves 4GB.
• Custom Kernels: Need to enable specific modules for iptables or upgrade to the latest 3.x kernel while the host runs 2.6? With KVM, you can.

Diagnosing the "Noisy Neighbor": The Steal Time Metric

How do you know if your host is overselling CPU? Look at %st (steal time) in top. This metric measures the time your virtual CPU waits for the real physical CPU to service it because the hypervisor is busy serving another VM.

Open your terminal and check:

$ top -b -n 1 | grep Cpu
Cpu(s): 12.5%us,  3.2%sy,  0.0%ni, 80.1%id,  0.2%wa,  0.0%hi,  0.1%si,  4.0%st

See that 4.0%st? That means 4% of your CPU cycles are being stolen by your neighbors. In a high-performance environment, anything above 0.5% is unacceptable latency. On CoolVDS KVM instances, we strictly manage host density to ensure this number stays at effectively zero.

Optimizing I/O for SSDs in 2012

With the price of SSDs finally dropping enough for enterprise adoption, we are seeing a shift from SAS 15k drives to solid-state storage. However, simply putting an SSD in a server isn't enough; you must tell the Linux kernel how to treat it.

The default I/O scheduler in CentOS 6 is often cfq (Completely Fair Queuing), which is optimized for spinning platters (HDDs). For SSDs, you want deadline or noop to reduce CPU overhead per I/O operation.

Here is how we configure our KVM guests for maximum throughput:

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

# Change to deadline (temporary)
$ echo deadline > /sys/block/vda/queue/scheduler

# Make it permanent in /boot/grub/grub.conf
kernel /vmlinuz-2.6.32-279.el6.x86_64 ro root=UUID=... elevator=deadline

This simple change can improve MySQL transaction throughput by 15-20% on virtualized SSD storage.

The Norwegian Context: Data Sovereignty and Latency

Beyond raw performance, location matters. Routing traffic from Oslo to a datacenter in Frankfurt or Amsterdam adds 20-40ms of round-trip latency. For real-time applications or VoIP services, that lag is noticeable.

Hosting within Norway also brings legal clarity. With the Personopplysningsloven (Personal Data Act) governing data privacy, keeping your customer data on Norwegian soil (under the jurisdiction of Datatilsynet) simplifies compliance significantly compared to navigating the complex US-EU Safe Harbor framework.

Pro Tip: If you are serving Norwegian users, trace the route to your server IP. If it hops through Sweden or Denmark, you are losing speed. CoolVDS peers directly at NIX (Norwegian Internet Exchange) in Oslo to keep latency typically under 5ms within the country.

Configuration: Tuning MySQL 5.5 on KVM

Since KVM guarantees your RAM, you can be aggressive with your database configuration. Unlike OpenVZ, where the OOM (Out of Memory) killer might snipe your MySQL process if the host node gets tight, KVM respects your allocation.

For a standard 4GB RAM KVM instance, here is a production-ready snippet for /etc/my.cnf to ensure InnoDB utilizes your memory effectively:

[mysqld]
# InnoDB Settings for 4GB RAM VPS
innodb_buffer_pool_size = 2G
innodb_log_file_size = 256M
innodb_flush_log_at_trx_commit = 2
innodb_flush_method = O_DIRECT

# Connections
max_connections = 300

# Query Cache (Be careful with high write loads)
query_cache_size = 32M
query_cache_limit = 1M

Note: Setting innodb_flush_method = O_DIRECT is particularly effective on our storage backend as it bypasses the OS cache, reducing double-buffering.

Conclusion: Choose Architecture, Not Just Price

Virtualization is the foundation of your digital presence. While containers have their place for lightweight testing, production demands isolation. The predictability of KVM, combined with the raw speed of SSD storage, provides the stability serious sysadmins require.

Don't let legacy spinning disks or noisy neighbors compromise your uptime. If you are ready for consistent I/O and strictly enforced resource limits, it is time to upgrade.

Ready to test the difference? Deploy a KVM instance on CoolVDS today and see your %st drop to zero.