Databases Reference
In-Depth Information
server is assigned to a specii c physical host server. Virtual servers cannot be allocated and use
physical server resources, such as memory, from multiple physical host servers.
EXTENDED FEATURES OF VIRTUALIZATION
Now that you are familiar with some of the fundamental concepts of virtualization, this section
looks at some of the more advanced features and capabilities the technology offers. This is where the
unique magic of the technology begins to appear, as some of these concepts simply weren't available
to traditional physical servers for all the time we were using them. While a hypervisor's primary
function is to “run” a virtual server and grant it the resources it requires as it needs them, the cur-
rent versions of VMware and many of Microsoft's server virtualization products also provide many
of the features discussed in the following sections.
Snapshotting
Snapshotting a virtual server is very similar to how SQL Server's own snapshot function works. In
principle, the hypervisor suspends the virtual machine, or perhaps requires it to be shut down,
and places a point-in-time marker within the virtual machine's data i les. From that point on, as
changes are made within the virtual machine's virtual hard drive i les, the original data is written to a
separate physical snapshot i le by the hypervisor. This can have a slight performance overhead
on the I/O performance of the virtual server and, more important, require potentially large
amounts of disk space because multiple snapshots can be taken of a virtual server, each having its
own snapshot i le capturing the “before” version of the data blocks. However, a copy of all of the
pre-change data gets saved to disk.
Having these snapshot i les available to the hypervisor enables it, upon request, to roll back all the
changes in the virtual server's actual data i les to the state they were in at the point the snapshot was
taken. Once completed, the virtual server will be exactly in the state it was at the point in time the
snapshot was taken.
While this sounds like a great feature which can offer a level of rollback functionality, it is
un-supported by Microsoft for use with virtual servers running SQL Server. Microsoft gives more
information about this in the Knowledge Base article 956893; however, until Microsoft supports its
use, snapshotting should not be used with virtual servers running SQL Server.
High-Availability Features
You read earlier that encapsulation means that a virtual server is ultimately just a collection of i les
stored on a i le system somewhere. These i les can normally be broken down into the virtual hard
drive data i les, as well as a number of small metadata i les that give the hypervisor information it
needs to “run” the virtual server, such as the CPU, memory, and virtual hard drive coni guration.
Keeping these i les in a centralized storage location — a SAN, for example — enables several
different host servers to access the virtual server i les. The trick that the i le system and hypervisor
have to perform is controlling concurrent read/write access to those i les in a way that prevents
corruption and two host servers running the same virtual server at once.
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