![]() The VSS writer contacts the Provider responsible for managing the volumes involved in creating the shadow copy and freezes all IO to the snap disk at a consistent point, by interfacing with all applications and the Windows operating system.The Requestor, or backup utility asks VSS to tell the Writers involved in the backup to gather their writer metadata documents (XML files that contain instructions for the backup) and send them to the Requestor.The Windows VSS Provider is a copy-on-write software solution but it is possible to replace this with a SAN based hardware solution. The VSS Provider maintains the shadow copies after they are initiated.The Windows system contains a VSS writer for basic filesystems, while third party database management systems will typically provide a specialised VSS writer for their databases. The VSS Writer is the tool that manages the snapshot capability of VSS and it will make sure that all the files in a snapshot are consistent.The VSS Requestor is typically a third party backup tool and it issues the VSS commands that create and manage the VSS snapshots.The VSS Coordination Service provides control and communication services between the other three components and is part of the Windows operating system.A working VSS system has four basic components, some of which can be provided by third party vendors. If an application is VSS aware, then the backup utilities will be able to process open files. The VSS freeze function can be used to take instant, and consistent backups, but for it to work effectively it needs to work with VSS aware applications and backup utilities. This previous versions feature was removed in Windows 2012, but re-instated in Windows 2016. VSS can be used to take regular snapshots of a disk, say every three hours, so that files can be easily wound back to previous states, should they become corrupted or deleted. These applications include backup and recovery products, data archive products with a requirement to take a copy of data frozen at a point in time and data mining applications. It is invoked by applications to take a snapshot of data, so processing can happen on a consistent set of data taken at a particular point of time. VSS is normally used in two different ways However the amount of disk space used for changes can vary, depending on whether the application rewrites the entire file when a change is made, or just changes parts of the existing file VSS backups will not require the same size disk space as the original data, unless all the files are completely overwritten. Only the changes are copied, not the entire file. This lot is explained in full in this link.įor the last two methods, VSS works by making a block-level copy of any changes that have occurred to files since the last shadow copy. The volume to be shadow copied does not need to be an NTFS volume, but at least one volume mounted on the system must be an NTFS volumeīoth hardware and software shadow copy providers create shadow copies by using one of the following methods:Ī complete copy is usually created by making either a 'split mirror' Copy-on-write, where the original data is copied to a different area when changes are made or Redirect-on-write, where changed data is redirected to a different volume. For the system provider, the shadow copy storage area must be on an NTFS volume. The native Windows system provider is one example of this. The advantage is that it can work with basic disks or logical volumes equally well, whereas a hardware implementation will just work with physical disks. There is also a system provider, which is a software provider that is built in to the Windows operating system.Ī hardware-based provider uses VSS to determine the point in time of the copy, to synchronise the data, flush buffers and manage the copy, but it passes this data to the underlying hardware, and the actual snapshot is performed by the storage array.Ī software-based provider creates the copies at software level, not hardware, and so uses operating system resources to maintain the shadow copy. There are two types of shadow copy providers: hardware-based providers and software-based providers. It is used in co-ordination with applications to provide point-in-time copies of single or multiple volumes. Volume Shadow Copy Service (VSS), often called Volume Snapshot Service, was introduced in Windows 2003.
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