VSS: Description, Compatibilities, and Troubleshooting Resources

This article explains the Volume Shadow Copy Service, and how it interacts with the Datto backup solution. Use this article as an aid when troubleshooting backup issues.

Environment

Datto ALTO
Datto SIRIS

Description

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 writer 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 to take an application-aware backup.
VSS Requestor: This is the writer that initiates the backup process.
VSS Provider: This writer 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.

For VSS backup solutions, only one process on the production machine can use the VSS writers at a time. If more than one of these processes are running in the same environment, a conflict between the solutions will happen, resulting in one or more VSS Writers failing.

Considerations

The VSS service will not run on windows volumes larger than 64TB. For more information on this refer to: Usability limit for Volume Shadow Copy Service (VSS) (external link)

The difference between VSS backups and DBD 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 the Microsoft Shadow Copy Provider's VSS writers encounter an error, the Datto Windows Agent will fail over to the Datto Snapshot Driver, referred to as the Datto Backup Driver, bypassing the VSS backup process. The Datto Backup Driver generates backups in a crash-consistent state. Data in processing at the time of the backup may not save correctly.

Crash-consistent backups are 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 fails, recent transactions for that database may not back up.

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

Common VSS Writers

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

vssadmin list writers

NOTE This command 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 of their associations to different processes:

ADAM (instanceName) 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: This writer handlesVMware Virtual Center Server Service for the ADAM instance VMwareVCMSDS.
ASR Writer: This writer is theAutomated System Recovery, which stores the BCD configuration of disks on the system on OSes (starting with Vista and Server 2008).
BITS Writer: This is the 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: The COM+ Class Registration Database Writer is responsible for the contents of the directory %SystemRoot%\Registration, which was first available in Vista and Server 2008 operating systems.
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: This is the writer responsible for backing up the Exchange Information Store as well as providing the ability to truncate Exchange transaction logs.
Event Log Writer: This writer isresponsible for protecting the Event Log Viewer contents.
FRS Writer: Starting with Server 2003, this writer is responsible for the File Replication Service.
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: This writer protects the configuration data for Internet Information Services (IIS).
IIS Metabase Writer: This is the writerresponsible for the Microsoft Internet Information Services, which is required by some SQL Server features.
Microsoft Exchange Writer: This writer is responsible for backing up the Exchange Information Store as well as providing the ability to truncate Exchange transaction logs.
Microsoft Hyper-V VSS Writer: This writer backs upHyper-V Virtual Machines.
MSSearch Service Writer: Starting with Server 2008, this writer exists to delete search index files from shadow copies after creation, 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, and 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: The NPS Writer isResponsible for protecting the Network Policy Server.
NPS VSS Writer: This writer protectsthe Network Policy Server.
NTDS: This is the 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: The Search VSS Writer is 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: This writer handles the Microsoft System Center Virtual Machine Manager.
Shadow Copy Optimization Writer: Beginning with Windows Vista and Windows Server 2008, this writer deletes specific files from volume shadow copies.
SMS Writer: This writer is responsible for the System Center Configuration Manager.
SPSearch VSS Writer: This writer is responsible for the SharePoint Server Search operation.
SQL Server Writer: This writer provides added functionality for backup and restore of SQL Server, including truncation of transaction logs. This writer only works 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: This writer is 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: Starting 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 identifies WMI-specific state and data during backup operations.

Troubleshooting Writers

For modern Windows systems VSS troubleshooting is often tied to the associated service of the writer, such as SQL or Exchange opposed to directly re-registering the binaries. Repairing the installation of these services can assist in fixing the health of the associated writer.

If the system writer or many VSS writers are unhealthy then overall health should be addressed.

Make sure that all available Windows Updates have been applied to the protected server in question
Perform an sfc scan on the protected machine to verify system health.
sfc /scannow
If this fails to repair the system, DISM may also be required.
DISM /Online /Cleanup-Image /RestoreHealth
When DISM is run, it is considered best practice to run the SFC scan again immediately after.
List the VSS writers from the command line after the repairs to see their state.
vssadmin list writers
If the writers look healthy, attempt another backup. After which, check the status of the writers again.

Missing writers

If any of the VSS writers are listed as "missing" it could be related to a permissions issue for files on the system. For more information on this type of error, please refer to Microsoft's documentation (external link).

Additional Resources

VSS Errors

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

	Acronis Nonstop Backup Service
	Acronis Scheduler2 Service
	Acronis Sync Agent Service
	Background Intelligent Transfer Service
	Backup Exec Error Recording Service
	Backup Exec PureDisk Filesystem Service
	Backup Exec Remote Agent for Windows
	Backup Exec VSS Provider
	Block Level Backup Engine Service
	Carbonite
	Datto Backup Agent Service
	DattoProvider
	EVault Software BUAgent
	Hyper-V Volume Shadow Copy Requestor
	Microsoft Azure Recovery Service (MARS)*
	Microsoft Software Shadow Copy Provider
	MozyHome Backup Service
	MozyPro Backup Service
	Replay 5 SnapToVM
	Replay Agent
	Replay Core
	Replay XML Command
	SAAZCore (Current RMM Version no longer requires this, can be removed)
	StorageCraft Raw Agent
	StorageCraft Shadow Copy Provider
	SQL Server (BKUPEXEC)
	SQL Server VSS Writer
	Volume Shadow Copy
	Windows Backup

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