Cloudware Technologies - Guide to Cloud Computing for Business and Technology Managers - page 308

shadow page table. This shadow page table points to the actual page frame
and is used by the hardware component called the memory management
unit (MMU) for dynamic address translation. Memory virtualization has
important implications on performance. VMMs use a range of optimization
techniques; for example, VMware systems avoid page duplication among
different virtual machines; they maintain only one copy of a shared page
and use copy-on-write policies, whereas Xen imposes total isolation of the
VM and does not allow page sharing. VMMs control the virtual memory
management and decide what pages to swap out; for example, when the ESX
VMware server wants to swap out pages, it uses a balloon process inside a
guest OS and requests it to allocate more pages to itself, thus swapping out
pages of some of the processes running under that VM. Then it forces the
balloon process to relinquish control of the free page frames.
There are two major types of hypervisors:
• Type I hypervisors run directly on top of the hardware. Therefore,
they take the role of an operating system and interact directly with
the ISA interface exposed by the underlying hardware, and they
emulate this interface in order to allow the management of guest
operating systems. These types of hypervisors are also called native
virtual machines since they run natively on hardware.
• Type II hypervisors require the support of an operating system to
provide virtualization services. This means that they are programs
managed by the operating system, which interact with it through
the ABI and emulate the ISA of virtual hardware for guest operating
systems. These types of hypervisors are also called hosted virtual
machines since they are hosted within an operating system.
15.1.3.2 VMM Solutions
A number of VMM solutions exist that are the basis of many utility or cloud
computing environments.
a. VMWare ESXi: ESXi is a VMM from VMWare. VMware is a pio-
neer in the virtualization market. Its ecosystem of tools ranges from
server and desktop virtualization to high-level management tools.
It is a bare-metal hypervisor, meaning that it installs directly on the
physical server, whereas others may require a host operating sys-
tem. It provides advanced virtualization techniques of processor,
memory, and I/O. Especially, through memory ballooning and page
sharing, it can overcommit memory, thus increasing the density of
VMs inside a single physical server.
b. Xen: Xen hypervisor started as an open-source project and has
served as a base to other virtualization products, both commercial