Databases Reference
In-Depth Information
The hypervisor's role is to coordinate the hosting and running of a number of virtual servers and
manage the allocation of the host server's physical resources between them. For example, on a
host server with 4 physical CPU cores, the hypervisor enables a number of currently running virtual
servers to behave as though each one has access to four physical CPU cores, known as virtual CPUs
(see Figure 17-2).
Hypervisor Manager
Windows Server 2008
R2
Hyper-V
Hypervisor
Hypervisor
Hypervisor
Hypervisor
Host Server
Host Server
Host Server
Host Server
Storage Area Network
FIGURE 17-2
What happens during periods of high workloads when there isn't enough physical CPU resource
to satisfy all of the virtual server requests for CPU time is perhaps one of the most performance
sensitive qualities of a hypervisor. The last thing you want is for virtual servers to become slow just
because one specii c virtual server is busy, although this problem has yet to be eliminated and can
still happen with some hypervisors.
How the hypervisor manages these situations varies between vendors. At a high level, they track how
much CPU time a virtual server has used recently, and use that data, along with system
administrator coni gured priority information known as shares or weighting, to determine in what
order a queue of requests for CPU time should be processed during periods of high demand.
VMware has an extra feature built into their hypervisor's CPU scheduling algorithms called relaxed
co-scheduling. The purpose of this is to identify which particular virtual CPUs in a multi-CPU
virtual server are the ones needing to do the work so it can avoid supplying un-required physical
CPU time to the virtual server; the principle being that lots of smaller workloads are easier to
i nd CPU resources for than a single large workload.
When installing VMware's server virtualization software, the hypervisor is installed directly on
the host server as its operating system; you don't, for example, install Windows i rst. Those who
deploy VMware's hypervisor will actually see a custom Linux installation boot to then run a set of
VMware services, but it's a self-contained environment that doesn't allow application software to
be installed. Meanwhile, users of Hyper-V will install a regular installation of the Windows Server
software and then add the Hyper-V role to the server. Installing this role is more than just adding
some components to the operating system; though, when the Hyper-V hypervisor gets installed it
actually becomes the server's operating system. The Windows installation that was installed on the
server now gets converted to become a virtual server that is run by the newly installed Hyper-V
hypervisor. This all happens transparently, but it is why Microsoft recommends not using the host
server's operating system for anything other than Hyper-V services.
Search WWH ::
Custom Search