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VSS Explained: Common Writers, Services, and Hotfixes available

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This article explains the Volume Shadow Copy Service, and how it is used with Datto devices.

Environment

  • Datto ALTO
  • Datto SIRIS

Overview

Index

What is the Volume Shadow Copy Service?

Microsoft's Shadow Copy Provider's Volume Shadow Copy Service, also commonly known as VSS, is used to take image-based backups. VSS is composed of the following:

  • VSS Writer: This tells the backup tool how to back up the application and its data.  A VSS writer must be present for the Volume Shadow Copy Service to quiesce (freeze) the program in order to take an application-aware backup.
  • VSS Requestor: This initiates the backup process.
  • VSS Provider: This allows the VSS backup process to work with the system's hardware and operating system.  The VSS provider is responsible for taking a snapshot. The VSS provider and its associated driver are required for this snapshot to take place.

Figure 1: VSS dependencies illustrated

For VSS backup solutions, it is important that only one process on the production machine use the VSS writers at a time. If more than one of these processes are running in the same environment, this will cause a conflict between the solutions, resulting in the failure of one or more VSS Writers.

What is the difference between VSS backups and STC backups?

On Windows systems, Datto's agent-based backup software uses the Microsoft Shadow Copy Provider to perform the initial step of the backup process. If it an error is encountered with the Microsoft Shadow Copy Provider's VSS writers, the following will occur (depending on which agent software is in use):

  • The Datto Windows Agent will fail over to the DattoProvider, referred to as the Datto Backup Driver.
  • The ShadowSnap agent will fail over to use the StorageCraft Shadow Copy Provider, referred to as the STC Backup Engine.

Both the Datto Backup Driver and the STC Backup Engine generate backups in a crash-consistent state. Data in processing at the time of the backup may not be saved properly.

  • This is a concern for systems running any services that include a VSS writer for backups, (such as database services). For instance, if the VSS writer for a database is failed, recent transactions for that database may not be backed up.

Application Aware VSS refers to the Microsoft Shadow Copy Provider. This VSS provider interacts with supported applications to provide backups as the the applications are running.

Common VSS Writers

To list current VSS writers on any Windows operating system, open the command prompt and type the command:

vssadmin list writers

This will not list all of the following VSS writers. In most cases, a Windows Desktop has 5 to 10 VSS writers, and a Windows Server machine has 10 to 15 vSS Writers.

Below is a list of the most commonly found VSS writers with a brief description about their individual associations to different processes: 

  • ADAM (instanceN) Writer: Beginning with 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.
  • ADAM (VMwareVCMSDS) Writer: VMWare Virtual Center Server Service for the ADAM instance VMWareVCMSDS.
  • ASR Writer: Automated System Recovery which stores the BCD configuration of disks on the system on OSes (starting with Vista and Server 2008).
  • BITS Writer: Background Intelligent Transfer Service Writer, first available in Vista and Server 2008. BITS uses the FilesNotToBackup registry key to exclude files from the BITS cache folder. The default cache location is %AllUsersProfile%\Microsoft\Network\Downloader\Cache.
  • Certificate Authority: This writer is responsible for enumerating the data files for the Certificate Server.
  • COM+ REGDB Writer: COM+ Class Registration Database Writer, responsible for the contents of the %SystemRoot%\Registration directory, which was first available in Vista and Server 2008 OSes.
  • DFS Replication Service Writer: Starting in Server 2003 R2, this writer protects Distributed File System Replication selections data.
  • DHCP Jet Writer: Starting with Server 2003, this writer is responsible for enumerating files required for the DHCP server role. This writer is not available for workstation OSes.
  • Exchange Writer: Responsible for backing up the Exchange Information Store as well as providing the ability to truncate Exchange transaction logs.
  • Event Log Writer: Responsible for protecting the Event Log Viewer contents.
  • FRS Writer: Responsible for the File Replication Service starting with Server 2003.
  • FSRM Writer: Starting with Windows Server 2003 R2, this writer enumerates the FSRM configuration files used for system state backup. During restore operations, it prevents changes in FSRM configuration and temporarily halts enforcement of quotas and file screens. This writer is not available for workstation OSes.
  • IIS Config Writer: Protects the configuration data for Internet Information Services (IIS).
  • IIS Metabase Writer: Responsible for the Microsoft Internet Information Services which is required by some SQL Server features.
  • Microsoft Exchange Writer: Responsible for backing up the Exchange Information Store as well as providing the ability to truncate Exchange transaction logs.
  • Microsoft Hyper-V VSS Writer: Responsible for backing up Hyper-V Virtual Machines.
  • MSSearch Service Writer: Starting with Server 2008, this writer exists to delete search index files from shadow copies after creation. This is to minimize the impact of Copy-on-Write I/O during regular I/O on these files on the shadow-copied volume.
  • MSDEWriter: 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. This writer is frequently called on by operations from the SQLSERVERAGENT service.
  • NPS Writer: Responsible for protecting the Network Policy Server.
  • NPS VSS Writer: Responsible for protecting the Network Policy Server.
  • NTDS: Active Directory Domain Services 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.
  • Search VSS Writer: Responsible for Search Operations
  • Performance Counters Writer: Starting with Windows 7 and Server 2008r2, this writer reports the performance counter configuration files. These files are only modified during application installation and should be backed up and restored during system state backups and restores.
  • Registry Writer: The registry writer is responsible for the Windows registry.
  • SCVMM Express Writer: Responsible for the Microsoft System Center Virtual Machine Manager.
  • Shadow Copy Optimization Writer: Beginning with Windows Vista and Windows Server 2008, this writer deletes certain files from volume shadow copies.
  • SMS Writer: Responsible for the System Center Configuration Manager.
  • SPSearch VSS Writer: Responsible for the SharePoint Server Search operation.
  • SQL Server Writer: Provides added functionality for backup and restore of SQL Server, including truncation of transaction logs. Works only with instances of Microsoft SQL Server.
  • System Writer: Beginning with Windows Vista and Windows Server 2008, the system writer enumerates all operating system and driver binaries.
  • Task Scheduler Writer: Starting with Windows 7 and Server 2008r2, this writer reports the Task Scheduler's task files.
  • TermServLicensing: Responsible for protecting the Microsoft Terminal Server Licensing.
  • VSS Metadata Store Writer: Starting with Windows 7 and Server 2008r2, This writer reports the writer metadata files for all VSS express writers.
  • WDS VSS Writer: Beginning with Windows Vista and Windows Server 2008, this writer reports the writer metadata files for all VSS express writers.
  • WINS Jet Writer: Beginning with Windows Server 2003, this writer is responsible for enumerating files required for WINS.
  • WMI Writer: Beginning with Windows Vista and Windows Server 2008, this writer is used for identifying WMI-specific state and data during backup operations.

Commonly Found VSS Related Services

The following chart lists commonly-found VSS-related services, and indicates which are compatible with the Datto solution.

You can view the VSS services installed on a Windows-based production machine by accessing the Windows Services Manager from the Start Menu, or by invoking the services.mc command from the Run prompt.

RedXMark.jpg  Acronis Nonstop Backup Service
RedXMark.jpg  Acronis Scheduler2 Service
RedXMark.jpg  Acronis Sync Agent Service
GreenCheck.jpg  Background Intelligent Transfer Service
RedXMark.jpg  Backup Exec Error Recording Service
RedXMark.jpg  Backup Exec PureDisk Filesystem Service
RedXMark.jpg  Backup Exec Remote Agent for Windows
RedXMark.jpg  Backup Exec VSS Provider
GreenCheck.jpg  Block Level Backup Engine Service
RedXMark.jpg Carbonite
GreenCheck.jpg Datto Backup Agent Service
GreenCheck.jpg DattoProvider
RedXMark.jpg EVault Software BUAgent
GreenCheck.jpg Hyper-V Volume Shadow Copy Requestor
GreenCheck.jpg Microsoft Software Shadow Copy Provider
RedXMark.jpg MozyHome Backup Service
RedXMark.jpg MozyPro Backup Service
RedXMark.jpg

Replay 5 SnapToVM

RedXMark.jpg

Replay Agent

RedXMark.jpg Replay Core
RedXMark.jpg Replay XML Command
RedXMark.jpg SAAZCore (Current RMM Version no longer requires this, can be removed)
GreenCheck.jpg ShadowProtect Service
GreenCheck.jpg StorageCraft Raw Agent
GreenCheck.jpg StorageCraft Shadow Copy Provider
RedXMark.jpg

SQL Server (BKUPEXEC)

GreenCheck.jpg

SQL Server VSS Writer

GreenCheck.jpg

Volume Shadow Copy

RedXMark.jpg

Windows Backup

Additional Resources

The following external knowledge articles provide resources for troubleshooting and optimizing VSS writers in the Windows environment.

Common VSS Troubleshooting Steps from Microsoft Answers

Windows XP

Windows 2003 (some are SP1 and some are SP2)

Windows Vista and Windows Server 2008 related

Windows Server 2008

Windows 7 and Windows 2008 R2 related


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