Databases Reference
In-Depth Information
Support for this largely comes from the i le
systems they use; VMware, for instance, has a
proprietary VMFS i le system that is designed
to allow multiple host servers to both read and
write i les to and from the same logical storage
volumes at the same time. Windows Server 2008
has a similar feature called Clustered Shared
Volumes that is required in larger Hyper-V
environments where multiple physical host servers
concurrently run virtual servers from the same
i le system volume. This is a departure from the
traditional NTFS limitation of granting only one
read/write connection access to an NTFS volume
at a time. Ensuring that a virtual machine is only started in one place at a time is controlled by
the hypervisors themselves. A system using traditional i le system i le locks and metadata database
updates is typically used to allow or prevent a virtual server from starting (see Figure 17-4).
Virtual
Server
Virtual
Server
Virtual
Server
Virtual
Server
Hypervisor
Hypervisor
Host Server
Host Server
SAN
FIGURE 17-4
By the way, while the cluster shared volumes feature of Windows sounds like a great solution
to numerous other requirements you might have, the technology is only supported for use with
Hyper-V. Microsoft warns of unintended and unexpected results if you try to use it for anything
else and you can i nd more information in the Microsoft TechNet article at
http://technet
Online Migration
After you have all the i les needed to run your virtual servers stored on some centralized storage,
accessible by multiple physical host servers concurrently, numerous features unique to virtualization
become available. The key differentiator here between the physical and virtual worlds is that you
are no longer dependent on a specii c physical server's availability in order for your virtual server to
be available. As long as a correctly coni gured physical host server with sufi cient CPU and memory
resources is available and it can access your virtual server's i les on the shared storage, the virtual
server can run.
The i rst of these features unique to virtualization is generically described in this chapter as online
migration, although Microsoft calls it Live Migration and VMware calls it vMotion for their imple-
mentations. Online migrations enable a virtual server to be moved from one physical host server to
another without taking the virtual server ofl ine.
For those unfamiliar with this technology and who
can't believe what they've just read, an example
should clarify the idea. In Figure 17-5, the virtual
server SrvZ is currently running on the physical
host server SrvA, while all of its i les are stored on
the SAN. By performing an online migration, you
can move SrvZ to run on SrvB without having to
shut it down, as shown in the second half of the
diagram.
Virtual
Server
SrvZ
SrvZ
Hypervisor
Hypervisor
SrvA
SrvB
SAN
FIGURE 17-5
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