Information Technology Reference
In-Depth Information
Linux-VServer, OpenVZ (Open VirtualiZation), FreeBSD Jails, and Solaris
Zones, based on Linux, FreeBSD, and Solaris, respectively, implement
operating system-level virtualization technologies. Operating system-level
virtualization allows a physical server to run multiple isolated operating
system instances, subject to several constraints; the instances are known as
containers, virtual private servers (VPSs), or virtual environments (VEs).
For instance, OpenVZ requires both the host and the guest OS to be Linux
distributions. These systems claim performance advantages over the sys-
tems based on a VMM such as Xen or VMware (there is only a 1%-3%
performance penalty for OpenVZ compared to a stand-alone Linux server).
15.1.3.1 Virtual Machine Monitor (VMM)
A virtual machine monitor (VMM), also called a hypervisor, is the software
that securely partitions the resources of a computer system into one or more
virtual machines. A guest operating system is an operating system that runs
under the control of a VMM rather than directly on the hardware: the VMM
runs in kernel mode, whereas a guest OS runs in user mode. VMMs allow
several operating systems to run concurrently on a single hardware plat-
form; at the same time, VMMs enforce isolation among these systems, thus
enhancing security. A VMM controls how the guest operating system uses
the hardware resources. The events occurring in one VM do not affect any
other VM running under the same VMM.
Thus, the VMM enables
• Multiple services to share the same platform
• The movement of a server from one platform to another, the so-called
live migration
• System modification while maintaining backward compatibility
with the original system
When a guest OS attempts to execute a privileged instruction, the VMM
traps the operation and enforces the correctness and safety of the opera-
tion. The VMM guarantees the isolation of the individual VMs and thus
ensures security and encapsulation, a major concern in cloud computing.
At the same time, the VMM monitors system performance and takes cor-
rective action to avoid performance degradation; for instance, the VMM
may swap out a VM (copies all pages of that VM from real memory to disk
and makes the real memory frames available for paging by other VMs) to
avoid thrashing.
A VMM virtualizes the CPU and memory. For instance, the VMM traps
interrupts and dispatches them to the individual guest operating systems.
If a guest OS disables interrupts, the VMM buffers such interrupts until the
guest OS enables them. The VMM maintains a shadow page table for each
guest OS and replicates any modification made by the guest OS in its own
Search WWH ::




Custom Search