Virtuozzo vs Xen
From WikiVS, the open comparison website
Virtuozzo and Xen are virtualization solutions that allow running multiple virtual environments within a single machine. These solutions are popular among VPS providers and are the two most popular solutions for Linux VPS providers. Most of the differences between Virtuozzo and Xen arise from the fact that Virtuozzo use OS-level virtualization, whereas Xen us paravirtualization.
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Virtualization Technology
Virtuozzo uses OS-level virtualization, meaning that the environment inside a VM is basically like a chroot. The kernel, iptables, and resource statistics are shared between VMs on the same machine. This means that VMs may not use another kernel, set up OS-level configuration, or display VM-specific resource statistics. CPU and IO scheduling, memory swap settings, and OS-level firewalls can only be modified on the host system and not within in the guest VM. VMs also use the same filesystem that the host system uses, albeit in a chrooted subdirectory, so the host has simple and open access to the guests' files.
Xen uses paravirtualization, which is the technology VMWare uses to virtualize its VMs [Not strictly true, VMWare takes a full-virtualisation - hardware emulation approach]. VMs running under Xen behave as if they are running on a dedicated box, so the kernel can be changed, swapiness can configured, and iptables can be modified. The VM is given virtual hardware to run its environment, and therefore, resource statistics look as if the VM is running on a dedicated box.
Supported Platforms
Virtuozzo allows both Windows and Linux guests. Xen works with Linux guests on all hardware but for Windows it's need hardware virtualization support in CPU level AMD-V and Intel VT.
Performance
The differing virtualization technologies provide differing performance benefits. Virtuozzo provides more total throughput than Xen because of lower overheads in its virtualization technology. Individual VMs with independent or competing interests, however, will fight for performance with the virtually free-for-all approach of Virtuozzo.
Total Performance Throughput
Xen runs all processes inside the VM with at least one extra layer of virtualized hardware, and thus, the virtualization adds overhead. Memory must be mapped another layer, CPU instructions must be wrapped, and disk access must go through the filesystem and disk driver of both guest and host systems.
Because Virtuozzo is almost nothing more than chroot, processes run almost unhindered and unvirtualized. Disk access is virtually unhindered since guest systems use the same filesystem as the host.
Performance of Individual VMs
While Virtuozzo allows more total throughput than Xen, individual VMs must invisibly compete for system resources in Virtuozzo if VMs have been allotted more resources than available, whereas Xen only provides hard caps for resource limits.
Burstable Resources
Virtuozzo allows resources to be burstable, and this is mostly seen with burstable RAM. While VMs may be provisioned a guaranteed minimum memory allotment, they may also use more than this amount when available. This allows for higher overall throughput since active VMs may use more resources while other VMs are inactive.
Xen's per-VM memory allotment is locked and may not be burstable.
Stability and Predictability
Because Xen provides a more thorough virtualization of a machine, it inherently provides more predictability than Virtuozzo. Also, because Xen does not share kernels, memory, and the filesystem between VMs, it provides more stability. Xen instances are known to crash less than Virtuozzo instances. This is because any kernel-level crashes in Virtuozzo will take down the host system, whereas Xen VMs have independent kernels.
Guaranteed Resources
Xen achieves stronger guarantees for resource allotment, especially with regards to memory. Although Virtuozzo also provides guaranteed memory, its provision for burstable memory makes the system less predictable. Processes fail more with out-of-memory errors in Virtuozzo than in Xen because of this issue.
VPS Practices
Some VPS providers like to use Virtuozzo because they can be oversold. Virtuozzo provides both guaranteed and burstable resources that actually produce higher throughput. However, Virtuozzo has license fees that VPS providers must pay. Some VPS providers use Xen because Xen instances provide more stability and predictability, which attract customers.
License
Virtuozzo is propietary software and must be licensed per VM. Xen is open-source and is free to use by anybody. There is an open-source alternative to Virtuozzo called OpenVZ that is meant to be similar to Virtuozzo
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