Virtual server clusters

Virtualization allows the execution on the same physical server of multiple operating systems each with its own applications.

For example, with virtualization, you can run on the same physical server:

• A Windows 2000 OS with a Microsoft SQL Server application
• A Windows 2003 OS with an Active Directory application
• A RedHat 3 Linux OS with an Apache/Tomcat application

We talk of three OS guests for the three virtual servers: Windows 2000, Windows 2003 and RedHat 3 Linux.

To implement virtualization, the physical server must run a hypervisor, which allows the creation and execution of virtual servers. The hypervisor itself generally runs inside an operating system, called the host OS, which manages resource sharing between the guest operating systems. Note that some hypervisors are bare metal hypervisors: they do not need a host OS to run.

Virtual server clusters

A cluster of virtual servers is characterized by:

• 2 physical servers with the same host OS
• A hypervisor installed in the host OS of both physical servers
• SafeKit installed in the host OS of both physical servers
• The SafeKit application module for the hypervisor (hyperv.safe, vmwareserver.safe...) on both physical servers

The SafeKit hypervisor application module will be a mirror module. This module will contain:

• The full replication of a virtual server. The configuration file of the module will define the replication of the directory that contains the guest OS (with its applications and data)
• The automatic restart scripts for the virtual server, defining the start and the stop of the guest OS in case of failure. Thus, if the primary physical server fails, the guest OS (with its applications and data) will be restarted on the secondary physical server

Each virtual server with its guest OS can be isolated in an individual SafeKit application module.

Thanks to SafeKit's multi-module capabilities, you can build architectures like active/active, with two physical servers in mutual takeover, or n-to-1, with n active physical servers and one single physical backup server.

Virtual server cluster example

The diagram shows a simple multi-module configuration with 2 virtual servers on a cluster of 2 physical servers:

• A mirror module for Virtual Server 1:
  • The active version of Virtual Server 1 is running on physical server 1
  • The SafeKit module replicates the full Windows guest OS, with its applications and data, to physical server 2
  • If physical server 1 fails, the SafeKit module will restart Virtual Server 1 on physical server 2
• A mirror module for Virtual Server 2:
  • The active version of Virtual Server 2 is running on physical server 2
  • The SafeKit module replicates the full Linux guest OS, with its applications and data, to physical server 1
  • If physical server 2 fails, the SafeKit module will restart Virtual Server 2 on physical server 1

Want to know more?

• Click here for the full list of modules delivered with SafeKit
• Click here to understand an alternative technique: application clustering
• Click here to learn how to choose between virtual server clustering and application clustering