Hyper-V Backups of Linux guests fail with VSS errors


Linux and other non-windows machine may report VSS errors when attempting to back up in Hyper-V using Host-level Backups


When running Hyper-V based backups on Linux guests they will fail with a general error about VSS problems.

Linux OSes do not have a VSS writer and by default Hyper-V will attempt to connect to the Guest and if integration services do not properly respond, Microsoft will interpret this as a VSS error.  This is not actually true as Linux does not use VSS, but due to compatibility barriers this issue may present. 


1) Only snapshot running Linux machines that are listed in Microsoft's Compatibility for Hyper-V for your host edition. If your distro or edition is not expressly supported by Microsoft Unitrends will not provide Hyper-V backup support for it, and more importantly you risk corrupting the VM by attempting backups.  More information can be found here:  Supported Linux and FreeBSD virtual machines for Hyper-V on Windows

2) Ensure your Linux machine Includes and supports Microsoft Linux Integration Services. If LIS is not installed or functional, Hyper-V will suspend your VM to perform the backup which will make it unavailable in production during the term of the backup and may result in inconsistent states for the operating system.  If your VM is listed to support LIS, but it is not functioning properly, you may require support from Microsoft or your Linux OS vendor in order to resolve the issue. Attempt agent-based backups to temporarily work around this issue. 

3) Do not make backups of Linux systems that run live databases like SQL or Oracle.  Though LIS handles general OS-level consistency, it does not perform application quiesce operations to ensure databases and certain other applications are in consistent states.  Even if your Linux OS is supported for VM Snapshot and Live motion by Microsoft that does not imply your applications deployed inside it are.  always consult 3rd party vendor documentation to ensure hypervisor snapshots are supported for your application. If you do not wish to do this required research, perform in-guest backups using traditional agents as if it was a physical server instead.  

4) Per Microsoft Documentation Under Storage > Linux Live Backup, Linux VMs only support Live backup when the VHD files for the VM are NOT deployed in SMB or iSCSI storage.  Local disk, Fiber Channel LUNs, or other supported direct attached storage.  If your Linux VM is in a remote mount or CSV using SMB3 or iSCSI, installation of an agent will be required to protect the machine if it cannot be moved to a compliant storage system.  

5) Virtual Machines that use passthru storage are not supported by hyper-V for host backup.  This limitation applies to all operating systems in hyper-V.  

6) Ensure your virtual machine meets all stated Microsoft Requirements for Linux including reviewing the Microsoft Best Practice documentation specific to your distribution provided by Microsoft. There are numerous Kernel, Grub, and other settings that are required in many cases to properly support Live backup as well as ensure stable operation. 

7) Ensure to plan for VM recovery considerations.  On recovery, if recovering to an alternate host, Microsoft may outside of Unitrends Controls upgrade the VM Generation or Configuration which may render a Linux VM unbootable. Recovery of linux VMs to a newer edition of hyper-V is not recommended.  This may also impact Linux operation in DRaaS environments. Additional understanding of the types of adapters, reserved IPs and static MACs may also need to be taken into consideration as if not configured to best practices they may need to be manually redone post-recovery.  



Hyper-V cannot successfully VSS snapshot a guest VM running Linux since Linux does not have a VSS writer.  Microsoft has other methods for Linux, and if not properly configured or supported by Microsoft the machine will fail to be backed up using Host backups.  

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