Before performing a virtual machine migration from Hyper-V or XenServer to vSphere, solutions providers need to take many factors into consideration. Virtual machine migration is a challenging process that can be riddled with incompatibility issues.
Virtualization expert Eric Siebert explains that before starting the virtual machine migration process, you need to be aware of potential setbacks, including problems with different virtual disk file formats and a lack of standards. Find out the proper way to format virtual machines (VMs) and virtual disk files and learn why there isn't an industry-standard VM file format.
Read Eric Siebert's answers to other frequently asked questions on virtual machine migration tools.
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•How difficult is a virtual machine migration from Microsoft Hyper-V or Citrix XenServer to VMware vSphere?
•Before performing a virtual machine migration, what incompatibilities should solutions providers be aware of?
•How do virtual disk files differ among the hypervisors?
•Is there a vendor-independent standard for VMs that solutions providers can refer to before performing a virtual machine migration?
•More resources on virtual machine migration and vSphere virtual machines
•About the expert
Virtual machines are encapsulated into a virtual disk file and several small companion files (i.e., BIOS, configuration), which makes for easy portability. However, migrating Hyper-V or XenServer VMs to vSphere is not as simple as copying those files to a vSphere server and powering on the VM. Because of incompatibilities among vSphere VMs and Hyper-V and XenServer VMs, you must first convert VMs to the format used by vSphere. Fortunately, a number of vSphere virtual machine migration tools exist to make this process fairly straightforward and easy.
The biggest incompatibility between Hyper-V, XenServer and vSphere is the virtual disk file, which uses different proprietary formats. Configuration files, which are text files that contain the settings and virtual hardware information for the VM, are also proprietary. VSphere uses .vmx files, and Hyper-V and XenServer use .xml files. In addition, there are other files such as BIOS and snapshots that have proprietary file formats. These different file formats make conversion necessary when going from one hypervisor to another.
VMware uses a proprietary Virtual Machine Disk (VMDK) format that it developed, and XenServer and Hyper-V use a proprietary Virtual Hard Disk (VHD) format. These formats are completely different from each other and cannot be used directly with another hypervisor. When performing a virtual machine migration, there are a number of tools and utilities that will convert virtual disks from one hypervisor format to another.
Because each vendor is competing with one another, there has not been a strong push to have a universal format that all hypervisors can use. An Open Virtualization Format (OVF) was proposed by all of the leading vendors but was more a universal method of packaging and distributing VMs than an actual virtual machine file standard. The OVF uses .xml files to describe the configuration of a VM and is not specific to virtual disks, which remain proprietary to each specific vendor. Today, the OVF is mainly used for importing and exporting VMs to any of the hypervisors that support it.
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About the expert
Eric Siebert is a 25-year IT veteran whose primary focus is VMware virtualization and Windows server administration. He is one of the 300 vExperts named by VMware for 2009. He is the author of the book VI3 Implementation and Administration and a frequent TechTarget contributor. In addition, he maintains vSphere-land.com, a VMware information site.