HP StorageWorks SAN Virtualization Services Platform Administrator Guide (5697-0934, May 2011)

8 Microsoft Volume Shadow Copy Service
The Volume Shadow Copy Service (VSS) captures and copies stable images for backup on running
systems, particularly servers, without unduly degrading the performance and stability of the services
they provide.
The VSS solution is designed to enable developers to create services (writers) that can be effectively
backed up by any vendor's backup application using VSS (requesters). A VSS requester has been
provided with the install to enable VSS to take advantage of unique SVSP features. Following is a
brief overview of VSS capabilities and features. More information is available from the following
link: http://msdn.microsoft.com/en-us/library/bb968832(VS.85).aspx.
The VSS model
The Microsoft Volume Shadow Copy Service (VSS) is a storage management interface for Microsoft
Windows Server 2003 and 2008. VSS enables your storage system to interact with third-party
applications that use the VSS application programming interface (API).
The SVSP VSS hardware provider is a Windows service (exe). The Microsoft VSS attaches to the
service and uses the service to coordinate the creation of snapshots (PiTs) on the virtual disks
presented by the DPM. You can initiate VSS-initiated snapshots through third-party backup tools
known as “requestors.”
NOTE: The VSS hardware provider does not work with synchronous mirrors.
The VSS model includes the following:
• The shadow copy mechanism. VSS provides fast volume capture of the state of a disk at one
instant in time—a shadow copy of the volume.
This volume copy exists side by side with the live volume, and contains copies of all files on
disk effectively saved and available as a separate device.
• Consistent file state through application coordination. VSS provides a COM-based, event-driven
interprocess communication mechanism that participating processes can use to determine
system state with respect to backup, restore, and shadow copy (volume capture) operations.
These events define stages by which applications modifying data on disk (writers) can bring
all their files into a consistent state prior to the creation of the shadow copy.
• Minimizing application downtime. The VSS shadow copy exists in parallel with a live copy
of the volume to be backed up, so except for the brief period of the shadow copy's preparation
and creation, an application can continue its work. The time needed to actually create a
shadow copy, which occurs between VSS Freeze and VSS Thaw events, typically takes about
one minute.
While a writer's preparation for a shadow copy, including flushing I/O and saving state,
may be nontrivial, it is significantly shorter than the time required to actually back up a
volume—which for large volumes may take hours.
• Unified interface to VSS. VSS abstracts the shadow copy mechanisms within a common
interface while enabling a hardware vendor to add and manage the unique features of its
own providers. Any backup application (requester) and any writer should be able to run on
any disk storage system that supports the VSS interface.
• Multivolume backup. VSS supports shadow copy sets, which are collections of shadow copies,
across multiple types of disk volumes from multiple vendors. All shadow copies in a shadow
copy set will be created with the same time stamp and will present the same disk state for a
multivolume disk state.
• Native shadow copy support. Beginning with Windows XP, shadow copy support is available
through VSS as a native part of the Windows operating system. As long as at least one NTFS
66 Microsoft Volume Shadow Copy Service