Backup Planning
When virtualization is used in an environment, there is often a decision to be made as
to whether backups will be taken from the virtualization host of the virtual machines,
or backup agents should still run within the virtual machines and backups will be
taken from inside the virtual machines. There is no “right” answer to which is the best
approach, but what is running inside the virtual machines and where you take the
backup can have a big impact on the granularity of any restore operations that are
performed.
Windows Server has long standardized on the Volume Shadow Copy Service (VSS),
which provides facilities that allow application vendors to write special VSS writers.
These application-specific modules, which are used to ensure that application data is
ready for backup, are registered with the operating system on which the application is
installed. All VSS writers registered on an operating system are called during a shadow
copy backup initiated by a VSS-aware backup program. The VSS writers ensure that all
data on disk for the application is in an application-consistent state and that other
writers are quiesced (which means paused during the operation) while the backup is
taken, maintaining the integrity of the on-disk data being backed up. An application-
consistent backup means that the data is in a suitable state to be restored and used
without corruption problems.
If a backup was taken at the Hyper-V host level of all virtual machine assets, primarily
the VHD files, then ordinarily the virtual machine would know nothing of the backup
being taken at the host level, so the data backed up would likely not be in an
application-consistent state. Hyper-V Integration Services includes a Backup (volume
snapshot) service, and this allows the Hyper-V host to notify each virtual machine
when a VSS backup is taken. The process then looks like the following and ensures
that backups of the virtual machines are in an application-consistent state:
1 . The backup software (the VSS requestor) on the Hyper-V server makes a request
for a VSS snapshot and enumerates the VSS writers (for example, the Hyper-V VSS
writer) on the system to ascertain that the data that can be backed up with VSS.
2 . The Hyper-V VSS writer (in conjunction with the VSS coordination service)
forwards the VSS snapshot request to each guest operating system via the Backup
integration service.
3 . Each guest operating system thinks it is receiving a native VSS request and
proceeds to notify all VSS writers on the guest to prepare for a snapshot.
4 . Each VSS writer in the guest operating systems writes any information to disk that
relates to its service (for example, Exchange and SQL) and notifies the VSS
coordinator that it is ready for a snapshot and tells it which data to back up
(although this part is ignored because we’ll be backing up the entire VHD from the
Hyper-V host).