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Windows Server 2012 : Managing virtual machines (part 1) - Optimizing workload consolidation

4/23/2014 9:26:13 PM
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1. Optimizing workload consolidation

One of the key benefits of server virtualization is that it allows businesses to better utilize their hardware resources. For example, in a traditional (nonvirtualized) environment, you might have a dozen servers at your head office location with each server performing some specialized role, such as file server, print server, database server, DHCP server, and so on. Many of these server roles can incur low average processing overhead, and this means that much of the system hardware in your server room might be significantly underutilized. For example, a dedicated DHCP server might have an average CPU usage of less than 10 percent.

By virtualizing your server workloads, however, you might be able to consolidate most—or even all—of your existing physical servers as virtual machines running on a single two-mode failover cluster of Hyper-V hosts. This reduces the number of physical servers in your environment from a dozen to only two, which not only simplifies your physical infrastructure but might also lead to significant cost savings because of the reduced power requirements. By using P2V conversion using the tools available in System Center Virtual Machine Manager or a third-party product, you now have a dozen virtualized server workloads to manage instead of a dozen physical systems. And by carefully analyzing the processor and memory requirements of your original servers, you can choose appropriate system hardware for your two Hyper-V hosts to ensure that they have enough processing and memory resources to run all these virtual machines with optimal performance.

By adjusting the memory allocated to each virtual machine on a host so that the virtual machine has just enough memory to perform optimally, you might be able to significantly increase the number of virtual machines that can simultaneously run on the host. Achieving higher virtual machine consolidation ratios through this approach can result in cost savings by eliminating the need for costly memory upgrades or even more costly purchases of additional host systems.

This capability is what Dynamic Memory is all about. Dynamic Memory was first introduced for Hyper-V in Windows Server 2008 R2 as a way of enabling Hyper-V hosts to make more effective use of physical memory allocated to virtual machines running on the host. Dynamic Memory works by adjusting the amount of memory available to a virtual machine in real time. These adjustments in memory allocation are based on how much memory the virtual machine needs and on how Dynamic Memory settings have been configured for the virtual machine.

Although it can provide some benefit when virtualizing server workloads, Dynamic Memory is especially valuable for virtual desktop infrastructure (VDI) scenarios, where it can provide scalability and performance benefits that can result in significant cost savings. This is because at any given time in a VDI environment, some of the virtual machines running on the host tend either to be idle or to have a relatively low load. By using Dynamic Memory, however, you can consolidate greater numbers of virtual machines on your Hyper-V hosts. This means that you’ll need fewer hosts for provisioning virtual desktops to your user population, which means you won’t need to procure as much high-end server hardware and can therefore save money.

Configuring Dynamic Memory

As shown in Figure 1, you can use the Memory page of the virtual machine settings dialog box in Hyper-V Manager to enable and configure Dynamic Memory for a virtual machine. Dynamic Memory can be enabled and configured only on a per-virtual machine basis. Once it is enabled for a virtual machine, the settings you can configure for Dynamic Memory include the following:

  • Minimum RAM This setting specifies the minimum amount of memory that the virtual machine can use while it is running.

  • Maximum RAM This setting specifies the maximum amount of memory that the virtual machine can use.

  • Memory buffer This setting specifies the amount of memory (as a percentage of the amount that the virtual machine actually needs to perform its workload) that can be allocated to the virtual machine when there is sufficient memory available on the host.

  • Memory weight This setting is a parameter that determines how available memory on the host is allocated among the different virtual machines running on the host.

Configuring Dynamic Memory for a virtual machine.
Figure 1. Configuring Dynamic Memory for a virtual machine.

Note

Understanding the memory buffer

The Memory Buffer setting for Dynamic Memory can be thought of as a memory reserve for the virtual machine. For example, if you configure the buffer to have a value of 50 percent, an additional amount of memory of up to 50 percent of the committed memory can be allocated to the virtual machine when needed.

One common scenario where this can provide benefits is when the virtual machine is running a workload that makes heavy use of the disk cache. In this case, increasing the buffer setting will result in additional memory being allocated to the virtual memory from the pool of idle memory on the host. For such scenarios, begin by raising the buffer setting from its default value of 20 percent to something like 30 or 40 percent and observe the change in the performance of the workload. Then if additional tuning is required, try raising the buffer setting a little further. Just don’t suddenly bump it up to 300 percent because you might end up starving the other workloads on the host!

The Minimum RAM setting is new in Windows Server 2012 Hyper-V, and it was introduced because Windows generally needs more memory when starting than it does when idle and running. As a result of this change, you now can specify sufficient Startup RAM to enable the virtual machine to start quickly and then a lesser amount of memory (the minimum RAM) for when the virtual machine is running. That way, a virtual machine can get some extra memory so that it can start properly, and then once it’s started, Windows reclaims the unneeded memory so that other virtual machines on the host can use the reclaimed memory if needed.

Another change in the way that Dynamic Memory can be configured in Windows Server 2012 is that now you can modify the Maximum RAM and Minimum RAM settings while the virtual machine is running. In Windows Server 2008 R2, the Maximum RAM setting could be modified only when the virtual machine was in a stopped state. This change gives you a new way of quickly provisioning more memory to a critical virtual machine when needed.

You can also use the Set-VM cmdlet to enable and configure Dynamic Memory for a virtual machine using Windows PowerShell. For example, let’s say you wanted to enable Dynamic Memory for a virtual machine named SRV-B that is running on HOST4 and configure the maximum RAM as 4 GBs. To do this, you first have to stop the virtual machine because you cannot enable or disable Dynamic Memory while the virtual machine is running. You can use the Stop-VM cmdlet to do this as follows:

PS C:\> Stop-VM -Name SRV-B -ComputerName HOST4

Next you can use the Set-VM cmdlet to enable Dynamic Memory for the virtual machine and set the maximum RAM to 4 GBs as follows:

PS C:\> Set-VM -Name SRV-B -ComputerName HOST4 -DynamicMemory -MemoryMaximumBytes 4GB

Now you can use Start-VM to restart the stopped virtual machine:

PS C:\> Start-VM -Name SRV-B -ComputerName HOST4

Finally, you can use Get-VM to verify the result:

PS C:\> Get-VM -Name SRV-B -ComputerName HOST4 | `
Format-List DynamicMemoryEnabled,MemoryMaximum

DynamicMemoryEnabled : True
MemoryMaximum : 4294967296

Let’s now say that you decide later that 3 GBs would be a better value for maximum RAM than 4 GBs. By using the –Passthru parameter, which specifies that an object is to be passed through to the pipeline, you can make the change and verify the result using a single Windows PowerShell command as follows:

PS C:\> Stop-VM -Name SRV-B -ComputerName HOST4 -Passthru | Set-VM -DynamicMemory `
-MemoryMaximumBytes 3GB -Passthru | Start-VM -Passthru | Get-VM | `
Format-List DynamicMemoryEnabled,MemoryMaximum

DynamicMemoryEnabled : True
MemoryMaximum : 3221225472

Configuring host reserved memory

Although Dynamic Memory can help you consolidate more virtualized workloads on a host, it’s important that the host itself have enough memory. If the host has insufficient memory, the management operating system might hang or crash, resulting in all virtual machines being stopped unexpectedly, with a possible loss of data occurring.

To prevent this from happening, Hyper-V automatically reserves some of the host’s physical memory for the management operating system. With Hyper-V in Windows Server 2008 R2 SP1, you could also manually specify how much memory should be reserved for the host by using Registry Editor to create a REG_DWORD registry value named MemoryReserve in the following location:

HKLM\Software\Microsoft\Windows NT\CurrentVersion\Virtualization

MemoryReserve could then be used to specify the amount of physical memory in MBs that should be reserved for the parent partition, with 2 GBs (2048 MBs) being the recommended value. The host then had to be rebooted for the change to take effect.

With Hyper-V in Windows Server 2012, however, the management operating system on the host now reserves more memory than in previous versions of Hyper-V to ensure greater reliability and responsiveness. In addition, manually configuring the MemoryReserve registry value as I just described can have undesirable results in some circumstances. For example, it might happen that a combination of management activity, background processes, scheduled tasks, and other activity on the host cause memory usage by the management operating system to temporarily rise above the value you manually configured using the MemoryReserve registry value. Should this occur, the management operating system might hang or crash, resulting in service interruption and possible data loss. As a result, Microsoft no longer recommends manually configuring the MemoryReserve registry value. Instead, you should allow Hyper-V itself to ensure enough memory is always reserved for the host.

 
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