SQL Writer Service
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 operating systems.
When running, Database Engine locks and has exclusive access to the data files. When the SQL Writer Service is not running, backup programs running in Windows do not have access to the data files, and backups must be performed using SQL Server backup.
Use the SQL Writer Service to permit Windows backup programs to copy SQL Server data files while SQL Server is running.
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 on a system continue to write to the volumes. The VSS provides a consistent interface that allows coordination between user applications that update data on disk (writers) and those that back up applications (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.
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 products for providing support for backup and restore operations. 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 the VDI API functions in order to perform the actual backups. Note that the VDI API is independent of VSS and is frequently used in software solutions that do not employ VSS APIs.
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 which in relies on VSS snapshots, such as System Center Data Protection Manager, as well as some other 3rd-party products, would be broken, or worse, at risk of taking backups of databases which were not consistent. If neither SQL Server, the system it runs on, nor the host system (in the event of a virtual machine), need to use anything besides Transact-SQL backup, then the SQL Writer service can be safely disabled and the login removed. Note that 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 elevated permissions in SQL Server because in the course of its activities it briefly freezes all I/O for the instance of SQL Server.
SQL Writer supports:
Full database backup and restore including full-text catalogs
Differential backup and restore
Restore with move
Auto-recovery of database snapshot
SQL Writer does not support:
File and filegroup backup
The SQL Writer service is a separate service from the SQL Server engine and is shared across different versions of SQL Server and across different instances of SQL Server on the same server. The SQL Writer service file ships as part of the SQL Server installation package and will be 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 will be replaced with the one from the installation package. Note that if the SQL Writer service was updated by a Service Pack or Cumulative Update and a RTM version of SQL Server is being installed, it is possible to replace a newer version of the SQL Writer service with an older one, provided that 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 will be 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.