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SQL Server 2012 : Running SQL Server in A Virtual Environment - VIRTUALIZATION CONCEPTS

7/18/2013 5:49:12 PM

Like any technology, virtualization covers a minefield of new terminology, features, and capabilities. To make things even more complicated, different vendors often use different terms for the same item. To help remove that ambiguity, this section covers the main terms and features commonly used by virtualization software currently being deployed.

Host Server

The host server, shown in Figure 1, is called the physical server deployed within the virtual environment. Today people use standard x64-based servers, such as an HP DL360, which are usually configured with a large number of CPU cores, large amounts of memory, some local disks for the hypervisor, and host bus adapters for access to storage area network (SAN) storage. The only difference between a host server and other servers is that its installed operating system’s only function is to manage the physical server’s resources to allow multiple virtual servers to run concurrently on the same physical hardware, rather than directly run application software such as SQL Server.




You will be familiar with the term virtualization software which we’ve used, and you will have seen how important that is to providing virtual servers. One of the components of that software is the hypervisor.

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 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 specific 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 configured 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 find 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 first. 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.

Virtual Server (or Guest Server or Virtual Machine)

The running of virtual servers, also called guest servers or virtual machines, is the sole purpose of a virtual environment. Each virtual server has very similar properties to a traditional physical server in that it will have a number of virtual CPUs, an amount of memory, and a quantity of virtual hard drives assigned to it. “Inside” the guest server, a regular operating system such as Windows Server 2008 will be installed on drive C: — just like a physical server would. Figure 3 shows a diagram representing the relationship between the hypervisor and the guest servers.



Inside virtual servers the hypervisor normally has a set of tools installed, often called client, or integration, services. These provide a level of integration between the virtual server and its hypervisor that wouldn’t otherwise be possible, such as sharing files between hypervisor and client or perhaps synchronizing the system time with the host server.

However, also installed is a driver that, on command from the hypervisor, can begin consuming specific quantities of memory within the virtual server.

It’s called a balloon driver because it inflates as needed to consume memory within the virtual server’s operating system. Its purpose is not to actually use the memory but to set it aside to ensure that nothing else within the virtual server is using it.

In comparison with the virtualization software and technology, there’s very little to say about virtual servers, and that’s a good thing, as the idea of virtualization is to make the fact they’re not running on a physical server invisible to them.

While virtual servers can be configured to “run” on different physical host servers using technologies like online migration that we’ll cover in the next section, at any point in time, a running virtual server is assigned to a specific physical host server. Virtual servers cannot be allocated and use physical server resources, such as memory, from multiple physical host servers.

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