SQL Writer service
Applies to: 
SQL Server - Windows only
The SQL Writer service provides added functionality for backup and restore of SQL Server through the Volume Shadow Copy Service framework.
The SQL Writer service is installed automatically. It must be running when the Volume Shadow Copy Service (VSS) application requests a backup or restore. To configure the service, use the Microsoft Windows Services applet. The SQL Writer service installs on all Windows operating systems.
Purpose
The Database Engine locks and has exclusive access to the database files. When the SQL Writer service isn't running, backup programs running in Windows don't have access to the data files, and backups must be performed using native SQL Server backup. Use the SQL Writer service to permit Windows backup programs to copy SQL Server database files while SQL Server is running and is maintaining exclusive locks on those files.
Volume Shadow Copy Service
The VSS is a set of COM APIs that implements a framework to allow volume backups to be performed while applications continue to write to those volumes. The VSS helps coordination between user applications that write data to disk (writers) and applications that back up that data (requestors).
The 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. For more information on the VSS, see your Windows documentation.
Note
When using VSS to backup a virtual machine that is hosting a Basic availability group, if the virtual machine is currently hosting databases that are in a secondary state, starting with SQL Server 2016 (13.x) SP2 CU2 and SQL Server 2017 (14.x) CU9 those databases will not be backed up with the virtual machine. This is because Basic availability groups do not support backing up databases on the secondary replica. Prior to these versions of SQL Server, the backup would fail with an error.
Virtual Backup Device Interface (VDI)
SQL Server provides an API called Virtual Backup Device Interface (VDI) that enables independent software vendors to integrate SQL Server into their backup and restore products. These APIs are engineered to provide maximum reliability and performance, and support the full range of SQL Server backup and restore functionality, including the full range of hot and snapshot backup capabilities. If a third-party vendor application requests a snapshot (VSS) backup, the SQL Writer service calls into the VDI API functions to perform the actual backups. The VDI API is independent of VSS and is frequently used in software solutions that don't employ VSS APIs.
Permissions
The SQL Writer service must run under the Local System account. The SQL Writer service uses the NT Service\SQLWriter login to connect to SQL Server. Using the NT Service\SQLWriter login allows the SQL Writer process to run at a lower privilege level in an account designated as no login, which limits vulnerability. If the SQL Writer service is disabled, then any utility that relies on VSS snapshots can break, and may risk taking backups of databases that aren't consistent. Examples include System Center Data Protection Manager, as well as some other third-party products.
If SQL Server, the system it runs on, and the host system (in the case of a virtual machine) don't need anything besides Transact-SQL backup, then the SQL Writer service can be safely disabled and the login removed. The SQL Writer service may be invoked by a system or volume level backup, whether the backup is directly snapshot-based or not. Some system backup products use VSS to avoid being blocked by open or locked files. The SQL Writer service needs sysadmin permissions in SQL Server because in the course of its activities it briefly freezes all I/O for the instance of SQL Server.
Features
SQL Writer supports:
- Full database backup and restore including full-text catalogs
- Differential backup and restore
- Restore with move
- Database rename
- Copy-only backup
- Autorecovery of database snapshot
SQL Writer doesn't support:
- Log backups
- File and filegroup backup
- Page restore
Service upgrade and maintenance
The SQL Writer service is a separate service from the SQL Server Database Engine and is shared across different versions and instances of SQL Server on the same server. The SQL Writer service file ships as part of the SQL Server installation package and is marked with the same version number as the SQL Server engine it ships with.
When a new instance of SQL Server is installed on a server or an existing instance is upgraded, if the version number of the instance being installed or upgraded is higher than the version number of the SQL Writer service that is currently on the server, that file is replaced with the one from the installation package.
If the SQL Writer service was updated by a Service Pack or Cumulative Update and newer version of SQL Server is being installed, you can replace a newer version of the SQL Writer service with an older one, as long as the installation has a higher major version number. For example, the SQL Writer service was updated in SQL Server 2016 (13.x) SP2 CU2. If that instance is upgraded to SQL Server 2017 (14.x) RTM, the updated SQL Writer service is replaced with an older version. In this case, you would need to apply the latest CU to the new instance in order to get the newer version of the SQL Writer service.