5 . The Backup integration service for each VM tells the Hyper-V VSS writer that it is
ready for a snapshot to be taken, and the Hyper-V VSS writer notifies the backup
application via the VSS coordinator that it is ready for a snapshot.
6 . The backup software takes a VSS snapshot of the filesystem containing the virtual
configuration files and the virtual hard disks, and all data on the virtual hard disks
is consistent, thanks to the VSS request being passed into the virtual machines.
Once the snapshot is taken, the VSS writer notifies the Hyper-V guests that the
snapshot is complete and they continue their normal processing.It should be noted that only VHD/VHDX content will be backed up using this method.
If a virtual machine has pass-through storage, has iSCSI storage connected through
the guest OS iSCSI Initiator, is connected to storage via Fibre Channel, or, prior to
Windows Server 2016, is a shared VHDX (but Windows Server 2016 adds support for
host-level backup of Shared VHDX, now known as VHD Sets), then that content would
not be backed up via a backup at the Hyper-V server level through the Hyper-V VSS
writer and would instead need to be backed up from within the guest virtual machine.
The preceding scenario describes an online backup, also known as a child VM
snapshot, where the guest operating system meets the following requirements:
The integration services are installed with the Backup integration service enabled.
The operating system supports VSS.
NTFS filesystems with basic disks (not dynamic) are used.If you have guest operating systems that use dynamic disks, that use non-NTFS
partitions, that don’t have the integration services installed, or that don’t have the
Backup integration service enabled or it’s just not supported (Windows 2000), then an
offline backup will be taken of the virtual machine, also known as a saved state
backup. This is because virtual machines that can’t support an online backup are
placed into a saved state during the VSS snapshot, which means that there is a period
of downtime for the virtual machine during the backup. Operating systems that have
to use saved state include Windows 2000, Windows XP, and Windows NT 4. Windows
2003, 2008, Vista, and above all support the online backup method with no virtual
machine downtime.
Prior to Windows Server 2012 R2, a Linux system had to be backed up using an offline
backup. Windows Server 2012 R2 introduced new features for Linux virtual machines,
one of which is live backup of Linux VMs. This is achieved through a new filesystem
snapshot driver that runs inside the Linux guest virtual machine. When a backup is
performed on the Hyper-V host, the filesystem snapshot driver is triggered in the
guest, which enables a filesystem-consistent snapshot to be taken of the VHDs that
are attached to the Linux VM. It should be noted that this is a different experience
from that available for Windows VMs, which provide filesystem-consistent and
application-consistent backups because applications have VSS writers that ensure that
application data is consistent on the disk. This is because there is not a standardized
VSS infrastructure in Linux, so there’s no way to ask applications to make their data