Recovering Data from failed Virtual Machines.
A virtual machine, commonly abbreviated to, VM is a software implementation of a computing environment in which an operating system (OS) or program can be installed and run.
If you set up and use virtual machines, The most efficient performance configuration is to run your virtual machines from separate physical hard drives:
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The virtual machine typically emulates a physical computing environment, but requests for CPU, memory, hard disk, network and other hardware resources are managed by a virtualization layer which translates these requests to the underlying physical hardware.
VMs are created within a virtualization layer, such as a hypervisor or a virtualization platform that runs on top of a client or server operating system. This operating system is known as the host OS. The virtualization layer can be used to create many individual, isolated VM environments.
Typically, guest operating systems and programs are not aware that they are running on a virtual platform and, as long as the VM's virtual platform is supported, this software can be installed in the same way it would be deployed to physical server hardware. For example, the guest OS might appear to have a physical hard disk attached to it, but actual I/O requests are translated by the virtualization layer so they actually occur against a file that is accessible by the host OS.
Advantages of Virtual Machines.
Virtual machines can provide numerous advantages over the installation of OS's and software directly on physical hardware. Isolation ensures that applications and services that run within a VM cannot interfere with the host OS or other VMs. VMs can also be easily moved, copied, and reassigned between host servers to optimize hardware resource utilization. Administrators can also take advantage of virtual environments to simply backups, disaster recovery, new deployments and basic system administration tasks. The use of virtual machines also comes with several important management considerations, many of which can be addressed through general systems administration best practices and tools that are designed to managed VMs.
Note: This entry refers to the term virtual machine (VM) as it applies to virtualization technology which creates independent environments for use by operating systems and applications which are designed to run directly on server or client hardware. Numerous other technologies, such as programming languages and environments, also use the same concepts and also use the term "virtual machine".
VM Application Monitoring.
A part of the VM application suite is a monitoring utility. VM Monitoring restarts individual virtual machines if their VMware tools activity is not received within a set time. Similarly, Application Monitoring can restart a virtual machine if the application task response times -out after a set time. You can generally enable these features and configure the sensitivity associated with non-responsiveness.
When you enable VM Monitoring, the VM Monitoring service (using VMware Tools) evaluates whether each virtual machine in the cluster is running by checking for regular heartbeats and I/O activity from the VMware Tools process running inside the guest. If no heartbeats or I/O activity are received, this is most likely because the guest operating system has failed or VMware Tools is not being allocated any time to complete tasks. In such a case, the VM Monitoring service determines that the virtual machine has failed and the virtual machine is rebooted to restore service.
Occasionally, virtual machines or applications that are still functioning properly stop sending heartbeats. To avoid unnecessary resets, the VM Monitoring service also monitors a virtual machine's I/O activity. If no heartbeats are received within the failure interval, the I/O stats interval (a cluster-level attribute) is checked. The I/O stats interval determines if any disk or network activity has occurred for the virtual machine If not, the virtual machine is reset.
Users can generally configure the level of monitoring sensitivity.