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What Should I Consider when Virtualizing a Physical Server?

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What Should I Consider when Virtualizing a Physical Server?

When you’re considering moving from a physical server to a virtual environment, what are some of the things you should consider? What type of host should you use? How large doe ...

When you’re considering moving from a physical server to a virtual environment, what are some of the things you should consider? What type of host should you use? How large does it need to be?  How many servers will you be virtualizing?

There is overhead in virtualizing an existing physical server, and the percentage varies according to workload. A good rule of thumb is to allocate an extra 10-25% of CPU, disk, and memory resources to accommodate the overhead of a virtual appliance as well as the overhead of Hyper-V itself.

It is recommended that you mirror the processor configuration for your virtual server, i.e., allocating a virtual processor for each physical processor on your server. For Hyper-V 2012, the maximum number of virtual processors allowed depends on the operating system of the server being virtualized. It can range from a low of 2 processors for Windows 2003 R2 or 4 for Windows 7, up to 32 for Windows 8.1 and 64 processors for Windows 2012 R2 and Redhat Linux 6.x, among others. See this link for more information.

Storage is another major consideration when virtualizing a physical server. You should allocate a virtual disk for each of the physical disks in your server, as noted. For vhd disks, the maximum size is 2TB, so you must create vhdx disks (supported on Hyper-V 2012 only) if you need to create a single disk larger than 2TB, as it supports disks up to 64TB in size. If Hyper-V integration services are not installed, disk I/O with IDE is significantly higher than with SCSI. You should make sure to install Hyper-V integration services so that SCSI controllers can be used. For maximum performance, create separate SCSI controllers for each virtual disk, and place each virtual disk on a separate physical disk.

Hyper-V virtual disks can be either fixed sized or dynamically expanding. Fixed sized disks are pre-allocated on the physical system, while, as you would expect, dynamically expanding disks have a much smaller initial footprint and grow and the disk data grows. For example, an empty 100GB disk would occupy 100GB of space if a fixed disk but only around 2MB or so if dynamically expanding. There is a tradeoff of performance vs convenience when using these disks. You can allocate more disks space than you actually have if the disks are dynamically expanding, but system performance will be higher with fixed size disks.  Some system workloads are so performance sensitive that they need special configuration; for that reason, Hyper-V provides a passthrough disk setting in which the virtual machine bypasses the Hyper-V host altogether for maximal performance. The downside to this is that some core features which make virtualization so compelling such as snapshots and clustering are not supported with passthrough disks.

For system memory, as noted previously, there is a small amount of hypervisor overhead, so a physical system may need an additional 10-20% of virtual memory than physical memory. This varies significantly depending on the workload and configuration of the server itself, but you must make sure you have enough memory to run your virtual machine. It is suggested that if possible, you virtualize using a 64-bit operating system, as it supports vastly larger amounts of memory than a 32-bit operating system does.

There are many different sorts of networking options available for virtual systems. For example, you can configure all virtual machines on the same host to communicate with each over a private virtual network if they do not need to access the host or external network. It is recommended that you have more than one network card on the physical Hyper-V host and dedicate one network solely to server administration, with no virtual machines using this NIC. Several other valuable recommendations can be found here.

Once you’ve successfully virtualized some servers, you can easily protect them with the Unitrends product, either by agents installed directly in your virtual machines, or by installing the Windows agent on the Hyper-V host itself.  You can protect a mix of physical and virtual systems of many types of operating systems with the same Unitrends appliance. The Unitrends web site describes the physical appliances, virtual appliances, and replication solutions that can be used to provide a comprehensive level of protection for your environment.

Comments ( 2 )

  1. ReplyMarc K
    That's odd that Hyper-V in 2012 only allows 2 virtual processors for a Windows Server 2003 guest. We have 2003 VMs running on 2008 R2 with 4 virtual processors assigned.
    • ReplyLisa Campbell
      Hi, thanks for your comments; this was not specified as it should have been in the blog. Hyper-V 2008 R2 did not support 2003 with more than 2 virtual CPUs, but Hyper-V 2012 does. Hyper-V 2012 allows up to 64 virtual processors, and the "limit" is there to indicate that testing was done for these operating systems to their licensed maximum by Microsoft. So if your OS version supports 4 processors, Hyper-V 2012 will also.

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