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Virtualization
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Virtualization is a technique of dividing a single server into several Virtual Servers, according to hardware and/or software, CPU Time/Process scheduling and memory allocation. Users can install and use their own Operating Systems simultaneously on these virtual servers.
Simply put, it is a server that runs different guest Operating Systems like RedHat Linux, FreeBSD, Debian, Microsoft Windows XP…etc at the same time, controlled by the Host Operating system. Users feel they are using their individual servers with the Operating System they wish to use.

Host OS - The base operating system which controls and co-ordinates the Virtual Servers
Guest OS - The Operating System installed on a Virtual Server

Types of Virtualization

The most commonly using virtualization techniques are;

1. Operating system-level virtualization

This is the simple and most popular virtualization technique. In this method, the virtualization is performed based on a single host operating system. Only the same instance of the host Operating System can be used on the virtual Servers as guest Operating Systems.

Examples : OpenVZ, Linux-VServer

2. Paravirtualization

This technique is somewhat similar to OS level Virtualization, except it is possible to use different guest Operating Systems like Microsoft XP, RedHat Linux, SUSE…etc on the virtual servers. This requires customized versions of Operating systems as guest operating systems. i.e, we can’t use the currently available XP, FC6…etc on these virtual servers, but the specially designed versions of these operating systems.

Many OS vendors have started selling their modified versions to perform Paravirtualization.

Examples : XEN, UML

hypervisor - The hypervisor is an application (eg. Kernel, Host OS) that runs between the server hardware and the guest operating systems. In the case of XEN, a specially designed Linux kernel is the hypervisor.

3. Full virtualization

This is the ideal virtualization technique in which any Operating system is used as the host OS and guest OSs. i.e, we can use unmodified versions of OSs on the virtual servers.

Examples : XEN, Vmware, z/VM

Full virtualization using XEN

Actually XEN lies between paravirtualization and full virtualization. XEN integrates its hypervisor on a linux kernal, so that we have to compile and run this modified kernel on the server on which we wish to perform the virtualization. XEN also supports virtual servers management though graphical interfaces. Therefore it is better to install a full version of Linux on the server first, then compile xen-linux kernel, reboot the server using the xen-linux kernel to create and manage servers. In this case, we can create and manage virtual servers through console and/or GUI oriented applications.

Implementing Full Virtualization using XEN (Simplified Steps)

1. Requirements
2. CPU support
3. Installing XEN
4. Enabling XEN Kernel
5. Installing Windows 2003 on top of a Virtual Machine
6. Installing Linux based OSs on top of a Virtual Machine

Requirements

The requirement of a XEN Fully Vitualized server are as follows;

1. A server installed with working Fedora 7 Operating System
2. Application builders like gcc v3.2.x binutils, GNU make
3. 1 GB Memory (Recommending 256 M for each virtual servers) 40 GB HDD
4. A VT enabled Intel system or AMD-V enabled AMD system (For Full Vitualization)
5. CDROMs of Guest Operating Systems (Linux OSs or Windows OSs like 2003/XP/Vista)

CPU Support

Make sure that the CPU supports Full virtualization over XEN.

1. For INTEL Systems, it should have the ‘vmx’ support. To check, use the following command;

   # grep vmx /proc/cpuinfo

2. For AMD systems, it should have ’svm’ support. To check, use the following command;

   # grep svm /proc/cpuinfo

   If these commands return nothing, then check whether the setting related to ‘Virtualization’ has enabled on the BIOS (it is turned OFF by default). If the BIOS is not enabled for virtualization, then it may be necessary to upgrade the BIOS.

Installing XEN

First, check for a XEN enabled kernel on the Host OS. Installation of a XEN kernel on Fedora 7 is quite simple using the ‘yum’ command as follows;

# yum install kernel-xen xen virt-manager

This will install,

1. XEN enabled kernel for both Host and Guest OSs
2. All required packages and their dependencies
3. Hypervisor for controlling Guest OSs
4. XEN packages for interacting with the Hypervisor

Enabling XEN Kernel

1. Edit the file ‘/boot/grub/grub.conf’ and set the default kernel to ‘kernel-xen’
2. Restart the server
3. Type the command ‘uname -r’ and make sure that the running kernel is ‘kernel-xen’

Installing Windows 2003 on top of a Virtual Machine

Not so long ago, installing any OS (whether it is Linux or Windows) on top of XEN was quite a nightmare for system administrators. Now Fedora 7 provides a way to install Guest OSs in a few mouse clicks. Here is how to do it.

1. Log into Fedora 7 (Host OS) in X session
2. Goto Application menu and select ‘Virtual Machine Manager’ from ‘System Tools’
3. This will show the wizard to cerate/manage Virtual Machines
4. Select the ‘New Machine’ option from the ‘File menu’
5. Type a name for the new node, say ‘Windows2003′ (make sure there are no spaces on the node name), and click on the ‘Forward button’.
6. On the next window select ‘Full Virtualization’ and click on the ‘Forward button’.
7. Now locate the installation media, you can either use a CD-ROM or use an ISO image
8. Also select the type of Guest OS as ‘Windows 2003′
9. Assign a storage space for the Virtual Machine
10. Now select ‘default virtual network’ and click on the ‘Forward button’.
11. Now allocate memory and CPU for the new Virtual Machine (A minimum of 250 M of RAM and 1 VCPU)
12. Now the installation of Windows 2003 oven XEN will begin
13. Once the installation is completed, we can start using the Windows 2003 installed Virtual Machine.

Installing Linux based OSs on top of a Virtual Machine

Use the same steps described in the previous section, using proper CD-ROM or ISO image.

Comments

If you wish to create Virtual Machines using a bash shell, use the following command.

# /usr/sbin/virt-install

This method will not allow you to directly install the OS from a CD_ROM or from an ISO image, but it can from NFS, FTP, and HTTP locations. A detailed procedure for doing this can be found at xensource.com

OPENVZ VPS
1.1. What is VPS?

A virtual private server or virtual dedicated server (VPS or VDS)is a server run through virtualization. It is used to partition a single physical server into many isolated virtual private servers. Each virtual private server looks and behaves exactly like a real networked server system, complete with its own set of init scripts, users, processes, filesystems, etc.,It fills the gap between shared hosting and dedicated hosting.
1.2. What is openvz?

OpenVZ is an operating system-level virtulization technology based on the Linux kernel and operating system. OpenVZ allows a physical server to run multiple isolated operating system instances, known as Virtual Private Servers (VPS) or Virtual Environments (VE).

OpenVZ offers the least flexibility in the choice of operating system: both the guest and host OS must be Linux (although Linux distributions can be different in different VEs). However, OpenVZ’s operating system-level virtulization provides better performance, scalability, density, dynamic resource management, and ease of administration than the alternatives.

OpenVZ kernel is a modified Linux kernel which adds support for Virtual Environments (VE).So it is easy to create and configure a VPS using openvz.
2. Requirements
2.1. Software Requirements

The Hardware Node should run either Red Hat Enterprise Linux 3 or 4, or Fedora Core 3 or 4, or CentOS 3.4 or 4. The detailed instructions on installing these operating systems for the best performance of OpenVZ are provided in the next sections.

This requirement does not restrict the ability of OpenVZ to provide other Linux versions as an operating system for Virtual Private Servers. The Linux distribution installed in a Virtual Private Server may differ from that of the host OS.
2.2. Hardware Requirements

The Hardware Node requirements for the standard 32-bit edition of OpenVZ are the following:
The computer should satisfy the Red Hat Enterprise Linux or Fedora Core hardware requirements.
i)IBM PC-compatible computer.
ii)CPUs: Intel Celeron, Pentium II, Pentium III, Pentium 4, Xeon, or AMD Athlon CPU. The more Virtual Private Servers you plan to run simultaneously, the more CPUs you need.
iii)Memory: Atleast 128 MB of RAM.The more memory you have, the more Virtual Private Servers you can run. The exact figure depends on the number and nature of applications you are planning to run in your Virtual Private Servers. However, on the average, at least 1 GB of RAM is recommended for every 20-30 Virtual Private Servers.
iv)HDD: Atleast 4 GB of free disk space. Each Virtual Private Server occupies 400-600 MB of hard disk space for system files in addition to the user data inside the Virtual Private Server (for example, website content). You should consider it when planning disk partitioning and the number of Virtual Private Servers to run.
v)NIC: Network card will be either Intel EtherExpress100 (i82557-, i82558- or i82559-based) or 3Com(3c905 or 3c905B or 3c595) or RTL8139-based are recommended.
A typical 2-way Dell PowerEdge 1650 1u-mountable server with 1 GB of RAM and 36 GB of hard drives is suitable for hosting 30 Virtual Private Servers.
3.Installation And Configuration
3.1. Pre-Setup

The first step before starting installation is to set up the openvz yum repository.

# cd /etc/yum.repos.d

# wget http://download.openvz.org/openvz.repo

# rpm –import http://download.openvz.org/RPM-GPG-Key-OpenVZ

# yum update

Now create a separate hard disk partition having atleast 4GB of space and mount it in /vz
3.2. Kernel Installation

You can install the kernel using yum.

yum install ovzkernel[-flavor]

Here [-flavor] is optional, and can be -smp or -enterprise.

But it will be not good always.
So here you can compile an optimized kernel by yourself.
Before kernel compilation you may need to check the hardware type that installed in your server.

# cat /proc/cpuinfo

This will give you the information about processor.

# lspci

This will give you the list of main other hardwares installed in your system.
Now we can start to build the vps kernel from source. So we need to download a kernel source.

# cd /usr/src

# wget http://www.kernel.org/pub/linux/kernel/v2.6/linux-2.6.16.tar.bz2

Now to download the appropriate patch from openvz.

# wget http://download.openvz.org/kernel/devel/026test018.1/patches/patch-026test018-combined.gz

The kernel configs are also available from openvz.

#wget http://download.openvz.org/kernel/devel/026test018.1/configs/kernel-2.6.16-026test018-i686-smp.config.ovz

Let us start the buiding…

# tar xvjf linux-2.6.16.tar.bz2

# cd linux-2.6.16

# mv ../patch-026test018-combined.gz

# gzip -dc patch-026test018-combined.gz | patch -p1

# cp ../kernel-2.6.16-026test018-i686-smp.config.ovz .config

# make menuconfig

Now you can select the options depend on your server configuration.

# make all

# make modules_install

# make

# make install

Now the VPS host kernel was successfully compiled and installed. So go to configure the boot loader either (grub or lilo).
3.3. Boot Loader Configuration

If GRUB is used as boot loader, it will configure automatically. Lines similar to following will be added to the grub.conf file.

# cat /boot/grub/grub.conf

Now you can find the following lines in grub.conf

title Fedora Core (2.6.16-026test018)
root (hd0,0)
kernel /vmlinuz-2.6.16-026test018 ro root=LABEL=/ rhgb quiet
initrd /initrd-2.6.16-026test018.img

Now edit this file as follows. (It is not necessary and just for understanding)

# vi /boot/grub/grub.conf

title VPS-openvz(2.6.16-026test018)
root (hd0,0)
kernel /vmlinuz-2.6.16-026test018 ro root=LABEL=/ rhgb quiet panic=5
initrd /initrd-2.6.16-026test018.img

Now set the “default” value to the newly edited image (in most case it will be ‘0′). Then save grub.config.If you are installing a remote server please add the following to the grub.

# grub –no-floppy

grub> savedefault –default=0 –once

grub> quit

Don’t reboot the system now. We must need to configure some more files too.
3.4. Sysctl

We need to set the following contents in sysctl.conf for its good working

# vi /etc/sysctl.conf

Now add the following parameters.

net.ipv4.ip_forward = 1

net.ipv4.conf.default.proxy_arp = 0

net.ipv4.conf.all.rp_filter = 1

kernel.sysrq = 1

net.ipv4.conf.default.send_redirects = 1

net.ipv4.conf.all.send_redirects = 0

3.5. SELinux

SELinux should be disabled.

# vi /etc/sysconfig/selinux

Add the following line to this file.

SELINUX=disabled

3.6. Conntracks

In the stable OpenVZ kernels (those that are 2.6.8-based) netfilter connection tracking for VE0 is disabled by default. If you have a stateful firewall enabled on the host node you should either disable it, or enable connection tracking for VE0.

To enable conntracks for VE0 please edit the file /etc/modprobe.conf

# vi /etc/modprobe.conf

Now add the following.

options ip_conntrack ip_conntrack_enable_ve0=1

In kernels later than 2.6.8, connection tracking is enabled by default.
3.7. Rebooting Into VPS

Now reboot the server. If it is loaded successfully we can proceed to installing the user-level tools for OpenVZ
3.8. Install Utilities

Now we need to install three basic utility packages .
vzctl: it is used to perform different operations on the OpenVZ VPS (eg : create, destroy, start, stop, set parameters etc.)
vzquota: This package is used to manage the VPS quotas.
vzpkg: this package is used to work with OpenVZ templates
Let us install this packages as follows

# yum install vzctl

# yum install vzquota

# yum install vzpkg

Now check the virtual ethernet device

# ifconfig

If it is not there use the following command to make it up.

# ifconfig venet0 up

Now reboot the server

# reboot

3.9. Install OS Templates

Now you need to install at leaset one os template.

# yum install vztmpl-fedora-core

Also you need to download a template package for creating vps .

# cd /vz/template/cache/

# wget http://download.openvz.org/template/precreated/fedora-core-4-i386-default.tar.gz

Alternative: use precreated template cache

As an alternative to creating a cache using template metadata, you can use precreated template cache taken from Downloads » Templates » Precreated, or directly from download.openvz.org/template/precreated, or from one of the mirrors.

Precreated templates can be easily updated using the following algorithm:

1. create temporary VE based on template
2. update VE using OS-specific tools (yum, apt or similar)
3. pack VE as a new template

Examples of this procedure are described in details at Updating Ubuntu template, Updating Debian template, Fedora template update

In order to use precreated template cache files, download files for chosen OS distributions and place them as-is (no unpacking needed) to the /vz/template/cache directory.

4. Usages
4.1. Create VPS

First you need to select a vps id. The id 0 is used for the hardware node itself.

# vzlist -a

This command list all the vps in the host.

You can create a vps using the default template or you can define a template package and a configuration. The default creation is as follows,

# vzctl create 101

If you want to create a vps using a OS template as follows

# vzctl create 101 –ostemplate fedora-core-4 -i –config vps.basic

101: is the vp id
fedora-core-4: is the OS template
vps.basic: is the configurations defined in vps.basic.conf
4.2. Configure VPS

Now we need to configure our vps. In this process we need to set up the following parameters.
i) Set the startup parameters
ii) Set the network parameters
iii) Set the root(user) password
So do the following commands in the host server.

# vzctl stop 101

# vzctl set 101 –onboot yes –save

# vzctl set 101 –hostname cyborg.com –save

# vzctl set 101 –ipadd 192.168.1.169 –save

# vzctl set 101 –nameserver 192.168.1.9 –save

# vzctl set 101 –userpasswd root:qwerty

# vzctl start 101

Now our vps will automatically start at the boot time of host. Also it have the host name “cyborg.com” with ip 192.168.1.169 and nameserver 192.168.1.9. The root password was set to “qwerty”. Now do the following

# vzlist -a
VEID NPROC STATUS IP_ADDR HOSTNAME
1 17 running 192.168.1.166 localhost
101 31 running 192.168.1.169 cyborg.com

4.3. Start,Stop and Restart

Now you need to do the following operations in your vps
i) start
ii)stop
iii)restart
iv)status
So run the following commands.

# vzctl stop 101
# vzctl start 101
# vzctl restart 101
# vzctl status 101

In my server it shows the following output.

# vzctl stop 101
Stopping VE …
VE was stopped
VE is unmounted

# vzctl start 101
Starting VE …
VE is mounted
Adding IP address(es): 192.168.1.169
Setting CPU units: 1000
Set hostname: cyborg.com
File resolv.conf was modified
VE start in progress…

# vzctl restart 101
Restarting VE
Stopping VE …
VE was stopped
VE is unmounted
Starting VE …
VE is mounted
Adding IP address(es): 192.168.1.169
Setting CPU units: 1000
Set hostname: cyborg.com
File resolv.conf was modified
VE start in progress…

# vzctl status 101
VEID 101 exist mounted running

4.4. Delete VPS

To delete a vps node we use the “destroy” command.

# vzctl stop 101

# vzctl destroy 101

Now the vps node 101 is deleted.You can check the status of this node. My server give the following output on the status operation.

# vzctl status 101
VPS 101 deleted unmounted down

5. Commands & Tools
5.1. Running Commands In VPS

We can run commands in a VPS through the host using “exec”.

# vzctl exec 101

An example is given below.

# vzctl exec 101 ifconfig
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING MTU:16436 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:0 (0.0 b) TX bytes:0 (0.0 b)

venet0 Link encap:UNSPEC HWaddr 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00
inet addr:127.0.0.1 P-t-P:127.0.0.1 Bcast:0.0.0.0 Mask:255.255.255.255
UP BROADCAST POINTOPOINT RUNNING NOARP MTU:1500 Metric:1
RX packets:27 errors:0 dropped:0 overruns:0 frame:0
TX packets:26 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:3118 (3.0 KiB) TX bytes:3720 (3.6 KiB)

venet0:0 Link encap:UNSPEC HWaddr 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00
inet addr:192.168.1.169 P-t-P:192.168.1.169 Bcast:192.168.1.169 Mask:255.255.255.255
UP BROADCAST POINTOPOINT RUNNING NOARP MTU:1500 Metric:1

5.2. OS Template Management

In this section we can see the list of cached and other templates.
To list the OS templates in the host please do the following commands.

# vzpkgls

To see the cached templates ,do the folowing

# vzpkgls –cached

To see the template used by a vps do the following,

# vzpkgls 101

The above commands give the following results in my server.

# vzpkgls
fedora-core-4-i386-default
fedora-core-4-i386-minimal

# vzpkgls –cached
fedora-core-4-i386-default

# vzpkgls 101
fedora-core-4-i386-default

5.3. Operations in VPS

To update the vps do the folllowing

# vzyum 101 update

To install a package (eg: php) do the following.

# vzyum 101 install php

To install an rpm(eg: MySQL-shared-3.23.57-1.i386.rpm) from the host ,do it as follows

# vzrpm 101 -ihv MySQL-shared-3.23.57-1.i386.rpm

6. Resource Management

This section is main important . The main goal of resource control in a VPS is to prevent a particular VPS from malicious or accidental usage of hardware resources.
6.1 Configuration Files

We can control the resource through a set of control parameters.All of these parameters placed in the openvz global configuration file or in the respective VPS configuration file.

The global configuration file is located in ” /etc/sysconfig/vz ” and the individual configuration file is located in ” /etc/sysconfig/vz-scripts/VPSID.conf ”
6.2 Disk Quota Management

There are a set of parameters determines disk quota in OpenVZ. The OpenVZ disk quota is realized on two levels:the per-VPS level and the per-user/group level. You can turn on/off disk quota on any level and configure its settings.
The main parameters are DISK_QUOTA, DISKSPACE,DISKINODES, QUOTATIME,QUOTAUGIDLIMIT.
DISK_QUOTA : Indicates whether first-level quotas are on or off for all VPSs or for a separate VPS.If is defined in the global configuration file (GF).

# grep DISK_QUOTA /etc/sysconfig/vz
DISK_QUOTA=yes

DISKSPACE : Total size of disk space the VPS may consume, in 1-Kb blocks.It is defined in the separate configuration file(SF).

# grep DISKSPACE /etc/sysconfig/vz-scripts/101.conf
DISKSPACE=”2000000:2200000″

DISKINODES : Total number of disk inodes (files, directories, and symbolic links) the Virtual Private Server can allocate.It is defined in the separate configuration file(SF).

# grep DISKINODES /etc/sysconfig/vz-scripts/101.conf
DISKINODES=”200000:220000″

QUOTATIME : The grace period for the disk quota overusage defined in seconds. The Virtual Private Server is allowed to temporarily exceed its quota soft limits for no more than the QUOTATIME period.It is defined in SF.

# grep QUOTATIME /etc/sysconfig/vz-scripts/101.conf
QUOTATIME=”0″

QUOTAUGIDLIMIT : Number of user/group IDs allowed for the VPS internal disk quota. If set to 0, the UID/GID quota will not be enabled.It is defined in SF.
Turning on/off per vps disk quota:Now to turning on per vps disk quota do the following.
Edit the separate configuration file

# vi /etc/sysconfig/vz-scripts/101.conf

Add the following,

DISK_QUOTA=yes

If you set the above value to “no”. The quota will be off.

# vzctl restart 101

# vzctl exec 101 df -h

Set up per vps disk quota:Now to set up per vps disk quota(eg : for a nod 102) we need to set up the following parameters DISKSPACE ,DISKINODES ,QUOTATIME

# vzctl set 102 –diskspace 1000000:1100000 –save

# vzctl set 102 –diskinodes 90000:91000 –save

# vzctl set 102 –quotatime 600 –save

# vzctl restart 102

# vzctl exec 102 df -h

Turning On/Off Second-Level Quotas for Virtual Private Server:The parameter that controls the second-level disk quotas is QUOTAUGIDLIMIT in the VPS configuration file. By default, the value of this parameter is zero and this corresponds to disabled per-user/group quotas.

Enabling per-user/group quotas for a Virtual Private Server requires restarting the VPS. The value for it should be carefully chosen; the bigger value you set, the bigger kernel memory overhead this Virtual Private Server creates. This value must be greater than or equal to the number of entries in the VPS /etc/passwd and /etc/group files.

# cat /etc/passwd|wc -l
55
# cat /etc/group|wc -l
66
# vzctl set 102 –quotaugidlimit 100 –save
# vzctl restart 102

Setting Up Second-Level Disk Quota Parameters:first to check the required packages are there in the vps.

# vzctl exec 102 rpm -q quota

Then ssh to the node 102
Now to edit the quota for the root do the following,

# edquota root

To report the quota do the folowing,

# repquota -a

This command gives the following output in my test vps.

# repquota -a
*** Report for user quotas on device /dev/simfs
Block grace time: 00:00; Inode grace time: 00:00
Block limits File limits
User used soft hard grace used soft hard grace
———————————————————————-
root — 455028 0 0 19878 0 0
smmsp — 8 0 0 2 0 0
named — 40 0 0 10 0 0
apache — 8 0 0 2 0 0
rpm — 9472 0 0 75 0 0
mysql — 1332 0 0 163 0 0

To check the quota stats do the following operation in the host server.

# vzquota stat 102 -t

6.3 CPU Sharing

We can set up the cpu utilization of a vps as follows.

# vzcpucheck

# vzctl set 102 –cpuunits 1500 –cpulimit 4 –save

# vzctl restart 102

DOWNLOADS

Linux kernel repository: http://www.kernel.org/pub/linux/kernel/
OpenVZ kernels,patches and configs: http://download.openvz.org/kernel/devel/
OpenVZ utilities: http://download.openvz.org/utils/
Os templates: http://download.openvz.org/template/
Other downloads: http://download.openvz.org
References

http://openvz.org/
http://forum.openvz.org/
http://blog.openvz.org/
http://wiki.openvz.org/

A Virtual Private Server or VPS (also known as a virtual dedicated server, a VDS, or virtual server) is a method of partitioning one physical server into multiple servers. Each one of these servers has the appearance and capabilities (with it’s own allocated resource) as it if were running it’s own dedicated server. Each VPS can run its own full-fledged operating system, and each server can be independently rebooted.

A virtual private server is the most economical way to get “guaranteed” resources for your web hosting environment.

VPS (Virtual Private Server) allows for better hardware utilization via lesser power utilization than a physical server.

However, maintenance of VPS servers may be marginally expensive. It might be difficult to make backup of one arbitrary software. Security issues are more complex in VPS providers than in dedicated server providers. There are two layers of security : both physical server security and VPS security. VPS monitoring might be more difficult than dedicated server monitoring.

VPS requires more expensive drivers like multi-port network adapters and more physical connections than dedicated server. There are some more management problems since you will use more Ethernet cables and network equipment like switches.

VPS providers tend to need more IP addresses than dedicated providers. If you have only 100 physical computers with 20 VPS on each it is still 2000 IP addresses. Physical systems that have more VPSs might have very high number of packets per second (PPS) though network. It is difficult to manage large number of IP addresses, MAC addresses etc.

There are sometimes software licensing advantages when running VPS. Some software licenses are by number of physical processors or physical computers and therefore they are cheaper when you can divide its price by number of VPS on one physical computer.

VPS performance is better nowadays with overhead less than 5% using modern virtualization software like Virtuozzo. But, disk I/O performances might have bottlenecks due to more disk fragmentation.

All in all, VPS technology should be selected after careful study of its advantages and disadvantages.

Most Popular technologies in the market are Xen, OpenVZ, VMWare.

OpenVZ is a complete server automation and virtualization solution.

OpenVZ is an Operating System-level server virtualization solution, built on Linux. OpenVZ creates isolated, secure virtual environments — VEs (otherwise known as virtual private servers, or VPSs) on a single physical server enabling better server utilization and ensuring that applications do not conflict. Each VE performs and executes exactly like a stand-alone server; VEs can be rebooted independently and have root access, users, IP addresses, memory, processes, files, applications, system libraries and configuration files.

Virtuozzo is a proprietary operating system virtualization product produced by SWsoft, Inc. Virtuozzo creates isolated virtual environments (VE) or containers on a single physical server and OS instance.

• Intelligent Partitioning - Division of a server into as many as hundreds of VEs with full server functionality.
• Complete Isolation - VEs are secure and have full functional, fault and
  performance isolation.
• Dynamic Resource Allocation - CPU, memory, network, disk and I/O can be changed without re-booting.
• Live Migration - Business continuity capabilities including live migration
  ensure data is available and recoverable.
• Mass Management - Suite of tools and templates for automated, multi-VE and multi-server administration.