Configuring Small Business Server 2011 in Hyper-V : Creating a Virtual Machine (part 2) - Machine Settings

REAL WORLD: Dynamic Memory Allocation

Windows Server 2008 R2 Service Pack 1 (SP1) and Microsoft Hyper-V Server 2008 R2 SP1 support dynamic memory for Hyper-V virtual machines. Dynamic memory allows you to allocate a minimum amount of memory to supported virtual machines, but also set a maximum amount of memory. The Hyper-V host can now dynamically assign memory to virtual machines based on the dynamic memory needs of the virtual machines and their overall memory weighting as set by the administrator. This dramatically enhances your ability to effectively use the resources of the Hyper-V host to support the maximum number of virtual machines without performance degradation.

When you configure the settings on a virtual machine, you enable dynamic memory for that machine and you set a Startup RAM and a Maximum RAM, along with a memory buffer percentage, and the memory weight of the virtual machine, as shown in Figure 6-14. The Hyper-V host first attempts to start all the virtual machines that are scheduled to start by allocating the Startup RAM specified to each VM. Once that RAM has been allocated, it looks at the buffer specified for each virtual machine and, if it can, reserves the full buffer to each virtual machine. If it is unable to assign each virtual machine the full buffer specified, it then uses a combination of the weighting of the memory for the virtual machines and the memory pressure of the virtual machines to allocate the available memory resources as effectively as possible. As the memory pressure from each virtual machine changes dynamically over time, the Hyper-V host will be allocated more or less RAM depending on the settings of the VMs on the host and the available RAM resources.

To use dynamic memory, your Hyper-V host must be running SP1 and your guest operating systems must be supported. For SBS networks, this means you must install Windows Server 2008 R2 SP1 on the Premium Add-on server if you have one, and upgrade the integration components on the virtual machine. Dynamic memory allocation is not supported for the main SBS server. Additional virtual machines running Windows Server 2003, Windows Server 2008, Windows Vista (Enterprise or Ultimate), or Windows 7 (Enterprise or Ultimate) are also able to take advantage of dynamic memory if they have been upgraded to the latest service pack.

REAL WORLD: Choosing Disk Types

Hyper-V supports three virtual disk types—dynamically expanding disks, fixed-sized disks, and differencing disks.

Dynamically expanding disks are created with a maximum size, and this is the size that the operating system of the VM sees. But the actual .vhd file of the disk takes up only as much space on your physical hard disk or array as absolutely required for the current contents of the VM drive. As the VM requires more storage space, Hyper-V automatically grows the .vhd file. This is very efficient, allowing you to add hard disk space only as absolutely required. But it does result in a slight performance hit every time the disk needs to grow, and more important, the .vhd file tends to become somewhat fragmented over time, also impacting performance. Nonetheless, we almost always use this type of disk for our nonproduction VMs. But not for the main SBS VM.

A fixed-size disk is also a .vhd file, but with a fixed-size .vhd file, the .vhd file is created at the full size on disk that it needs to be. It takes a significant amount of time to create the .vhd file, but it will be created as a contiguous file (or as contiguous as the underlying fragmentation of your physical disk or array allows).

A differencing disk is an interesting disk type. It is like a dynamic disk in that it gets only as large as it needs to. But a differencing disk is a great way to combine the disk space requirements of multiple VMs. You create the original “base” VM, and then mark the disks as read-only. You can actually delete the VM that created the base disks. Then you create one or more VMs that have the same operating system, and you create them with one or more differencing disks. The differencing disk points to the original base .vhd file, and the only thing that gets saved to the differencing disk is any change from the base VM. This allows multiple VMs to share the same base, simplifying deployment of different versions of the same base system—very useful for quickly building test networks.

The biggest disadvantage of differencing disks is their impact on speed. As more VMs point back to the original VHD files, the access to that VHD can be slowed. And if anything causes a change to the original VHD, all the VMs that point to it can be lost. Over time, the size advantage of differencing disks is also reduced as updates and service packs are applied to the differenced VMs. But for a test environment? Differencing disks can be a great speed and resource saver.

The final option is a pass-through disk that points directly to the physical partition. This is the fastest option, but it provides the least flexibility. You might choose this option if you are running Microsoft SQL Server on a VM and you have a performance-sensitive application. But even there, we tend to avoid it if at all possible. The difference in performance compared to a fixed-size VHD is small (on the order of 5 percent, we’re told). Spend your money on good disk I/O subsystems and avoid pass-through disks.