Volume Shadow Copy Service (VSS)

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. VSS has following main merits:

VSS provides fast volume capture of the state of a disk at one instant in time, i.e 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.
VSS minimize 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 Freeze and Thaw events, typically takes about one minute. While a writer's preparation for a shadow copy, including flushing I/O and saving state (see Overview of Pre-Backup Tasks), may be nontrivial, it is significantly shorter than the time required to actually back up a volume-which for large volumes may take hours.

Windows in-box VSS Writers (From MSDN):

Windows XP/Server 2003 in-box VSS Writers:

MSDEWriter

The MSDEWriter writer is a VSS Writer for SQL Server. MSDEWriter is included with the VSS framework in Windows XP and Windows Server 2003. MSDEWriter works with SQL Server 7.0, SQL Server 2000, and SQL Server 2005.

SqlServerWriter

The SqlServerWriter writer is a VSS Writer for SQL Server. SqlServerWriter is included with SQL Server 2005. SqlServerWriter only works with instances of SQL Server 2005.

Windows VISTA/Windows 7 in-box VSS Writers

Registry Writer

The registry writer is responsible for the Windows registry. Beginning with Windows Vista and Windows Server 2008, the registry writer now performs in-place backups and restores of the registry. On versions of Windows prior to Windows Vista, the registry writer used an intermediate repository file (sometimes called a "spit file") to store registry data.

In Windows Vista and later, the registry writer does not report user hives.

COM+ REGDB Writer--COM+ Class Registration Database Writer

Beginning with Windows Vista and Windows Server 2008, this writer is responsible for the contents of the %SystemRoot%\Registration directory. The COM+ catalog is a file in %SystemRoot%\Registration with a name that follows the pattern Rxxxxxxxxxxxx.clb, where xxxxxxxxxxxx is a 12-digit hexadecimal number. There can potentially be multiple such files if COM+ applications have been configured on the computer. The file that contains the current COM+ catalog is the file with the largest value of xxxxxxxxxxxx.

The COM+ class registration database depends on a registry key being backed up and hence needs to be backed up and restored together with the registry.

MSSearch Service Writer

Beginning with Windows Server 2008, this writer exists to delete search index files from shadow copies after creation. This is done to minimize the impact of Copy-on-Write I/O during regular I/O on these files on the shadow-copied volume.

To install this writer on the server, you must install the File Services role and enable the Windows Search Service.

WMI Writer

Beginning with Windows Vista and Windows Server 2008, this writer is used for identifying WMI-specific state and data during backup operations. The data includes files from the WBEM repository.

BITS Writer--Background Intelligent Transfer Service (BITS) Writer

Beginning with Windows Vista and Windows Server 2008, the BITS writer uses the FilesNotToBackup registry key to exclude files from the BITS cache folder. The default cache location is %AllUsersProfile%\Microsoft\Network\Downloader\Cache.

In addition, the BITS writer excludes the following files from backup: the BITS state files (qmgr0.dat and qmgr1.dat), the BITS debug log file, and BITS partially downloaded files.

If the BITS download destination file is an SMB file, the client account must have a trust relationship to the server, or else backups may fail.

ASR Writer--Automated System Recovery (ASR) Writer

Beginning with Windows Vista and Windows Server 2008, the ASR writer stores the configuration of disks on the system. This writer reports the Boot Configuration Database (BCD) and is also responsible for dismounting the registry hive that represents the BCD during shadow copy creation. The ASR writer must be included in any backups required for bare-metal recovery. For more information about ASR, see Using VSS Automated System Recovery for Disaster Recovery.

System Writer

Beginning with Windows Vista and Windows Server 2008, the system writer enumerates all operating system and driver binaries. This writer runs as part of the Cryptographic Services (CryptSvc) service. On Windows Vista and later, the system writer generates a file list that contains the following files.

All static files that have been installed. A static file is a file that is listed in the component manifest with the writeableType file attribute set to "static" or "". Static files include all files that are protected by Windows Resource Protection (WRP). However, not all static files are WRP-protected files. For example, game files are static but not WRP-protected so that administrators can change parental control settings.

The contents of the Windows Side-by-Side (WinSxS) directory, including all manifests, optional components, and third-party Win32 files.

Note Many of the files in the %windir%\system32 directory are hard links to files in the WinSxS directory.

All PnP files for installed drivers (owned by PnP).

All user-mode services and non-PnP drivers.

All catalogs owned by CryptSvc.

The restore application is responsible for laying down the files and registry and setting ACLs to match the system shadow copy. The appropriate hard links must also be created for a system state restore to succeed.

Shadow Copy Optimization Writer

Beginning with Windows Vista and Windows Server 2008, this writer deletes certain files from volume shadow copies. This is done to minimize the impact of Copy-on-Write I/O during regular I/O on these files on the shadow-copied volume. The files that are deleted are typically temporary files or files that do not contain user or system state.

Active Directory Lightweight Directory Services (LDS) VSS Writer

Beginning with Windows Server 2003, this writer reports the ADAM database file (adamntds.dit) and the associated log files for each instance in %program files%\Microsoft ADAM\instanceN\data, where N is the ADAM instance number. These database log files are required to restore ADAM instances.

Active Directory Domain Services (NTDS) VSS Writer

Beginning with Windows Server 2003, this writer reports the NTDS database file (ntds.dit) and the associated log files. These files are required to restore the Active Directory correctly.

If the computer that contains the NTDS database is a domain controller, the backup application should always perform a system state backup across all volumes containing critical system state information. At restore time, the application should first restart the computer in Directory Services Restore Mode and then perform a system state restore.

Dynamic Host Configuration Protocol (DHCP) Writer

Beginning with Windows Server 2003, this writer is responsible for enumerating files required for the DHCP server role. This writer is an in-box writer for Windows Server operating system versions; it does not ship in Windows Client.

File Server Resource Manager (FSRM) Writer

Beginning with Windows Server 2003 R2, this writer enumerates the FSRM configuration files that are used for system state backup. During restore operations it prevents changes in FSRM configuration and temporarily halts enforcement of quotas and file screens. After the restore is complete, the writer refreshes FSRM with the configuration that was restored. This writer is only present when FSRM (part of File Services role) is both installed and running. This writer is an in-box writer for Windows Server operating system versions; it does not ship in Windows Client.

Windows Internet Name Service (WINS) Writer

Beginning with Windows Server 2003, this writer is responsible for enumerating files required for WINS.

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