Hyper-V brings enhancements for Linux VMs

The latest version of Hyper-V includes enhanced support for Linux VMs, allowing for advanced features such as Dynamic Memory and live VM backup.

Over a period of time, Microsoft made some big changes to the Hyper-V role to provide features such as Dynamic Memory, Live VM Backup, Cloud backup, online VHDX resize and Generation 2 virtual machines, but those were designed primarily for Windows VMs. An organization might run several different operating systems in a production environment, including Linux distributions. With the initial release of Hyper-V, Microsoft supported a minimal feature set for VMs running Linux distributions by installing an add-on component called Linux Integration Services (LIS). More recently, Microsoft has been collaborating with vendors to add new features and eliminate the need for installing LIS on newer Linux distributions.

Dynamic Memory

This was a much-awaited feature for Linux VMs. Starting with Windows Server 2012 R2, hot addition and removal of memory are now fully supported for Linux VMs running on a Windows Server 2012 R2 Hyper-V host. Although you can make Dynamic Memory work for Linux VMs running on Windows Server 2012 Hyper-V hosts, officially it is not supported. Whenever Hyper-V needs to allocate memory to a Linux VM, it uses the hot-add function of the Dynamic Memory feature, which is actually implemented by the Hyper-V host. If Hyper-V needs to reclaim some memory from Linux VMs, it uses the ballooning driver installed as part of LIS. It is important to note that the Dynamic Memory feature is available only for 64-bit Linux distributions. Some Linux distributions might require you to enable support for this feature. For example, CentOS and Red Hat Linux distributions require that you enable the Dynamic Memory hot-add function by creating a "udev" rule entry under the /etc/udev/rules.d path.

Live VM backup

In earlier versions of Hyper-V, you had to suspend Linux VMs to create a backup. If you are running Linux VMs on Windows Server 2012 R2 Hyper-V hosts, these VMs can now be backed up without any downtime using compatible Hyper-V backup tools, such as Windows Server Backup. It is important to understand that since Windows VMs utilize the Volume Shadow Copy Service, or VSS, to initiate live backups, they can support two types of backups: file-system backups (VHD level) and application-consistent backups. For Linux VMs, Microsoft introduced a file-system snapshot driver that is capable of performing only file-system-based backups. It has also been reported, as a known issue, that a live backup of Linux VMs can fail if you have attached an iSCSI device or pass-through disk to them.

Support for Generation 2 VMs

Earlier it was not possible to install a Linux distribution as a Generation 2 VM. Starting with Windows Server 2012 R2, newer Linux distributions, such as CentOS and Red Hat Enterprise 7.0, and SUSE SLES 12 and Ubuntu 14.10, can be installed as Generation 2 VMs. It is also worth noting that a Generation 2 VM has secure boot enabled by default. Since Linux VMs do not support the secure boot option, you might want to disable it from the property page of the Linux VM or by executing a PowerShell command on the Hyper-V host.

Online backup

Online backup support is another new enhancement to Linux VMs running on Windows Server 2012 R2 Hyper-V hosts. You can now initiate online backups of running Linux VMs to Microsoft Azure using the Windows Azure Online Backup utility and System Center Data Protection Manager.

Online VHDX resizing

You can now resize fixed VHDX files attached to a Linux VM without any downtime. This is a feature many organizations have been waiting to see for Linux VMs because the online VHDX resizing feature eliminates the need to go through complex change management processes and helps increase the storage for critical Linux workloads without any downtime. It is worth mentioning that not all Linux distributions support the online VHDX resizing feature.

Copy file from host to Linux VMs

Windows Server 2012 R2 Hyper-V introduced an Integration Services component called Guest Services. Once Guest Services is enabled, it allows you to copy files to Linux VMs. Linux Integration Services provides a daemon called hv_fcopy. At the time of writing, the file copy functionality is supported only for CentOS and Red Hat 6.6 and 7.0, SUSE SLES 12, and Ubuntu 14.10 and 14.04.

Although Microsoft's primary focus has been to develop new Hyper-V features primarily for Windows VMs, many features are now available to Linux distributions and may bring huge value to the organizations looking to consolidate their Linux workloads to the Hyper-V server virtualization platform.

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