Merge pull request #6658 from bluefooted/master

Adding documentation for changes in SQL Writer behavior

Author: rmca14

docs/database-engine/configure-windows/sql-writer-service.md

@@ -41,6 +41,9 @@ manager: craigg
  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.  
+
+> [!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 [!INCLUDE[ssSQL15](../../includes/sssql15-md.md)] SP2 CU2 and [!INCLUDE[ssSQL17](../../includes/sssql17-md.md)] 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 [!INCLUDE[ssNoVersion](../../includes/ssnoversion-md.md)], the backup would fail with an error.
   
 ## Virtual Backup Device Interface (VDI)  
  [!INCLUDE[ssNoVersion](../../includes/ssnoversion-md.md)] provides an API called Virtual Backup Device Interface (VDI) that enables independent software vendors to integrate [!INCLUDE[ssNoVersion](../../includes/ssnoversion-md.md)] 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 [!INCLUDE[ssNoVersion](../../includes/ssnoversion-md.md)] backup and restore functionality, including the full range of hot and snapshot backup capabilities.  
@@ -71,4 +74,6 @@ manager: craigg
 
 -   Page restore  
 
-  
+## Remarks
+The SQL Writer service is a separate service from the [!INCLUDE[ssNoVersion](../../includes/ssnoversion-md.md)] engine and is shared across different versions of [!INCLUDE[ssNoVersion](../../includes/ssnoversion-md.md)] and across different instances of [!INCLUDE[ssNoVersion](../../includes/ssnoversion-md.md)] on the same server.  The SQL Writer service file ships as part of the [!INCLUDE[ssNoVersion](../../includes/ssnoversion-md.md)] installation package and will be marked with the same version number as the [!INCLUDE[ssNoVersion](../../includes/ssnoversion-md.md)] engine it ships with.  When a new instance of [!INCLUDE[ssNoVersion](../../includes/ssnoversion-md.md)] 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 [!INCLUDE[ssNoVersion](../../includes/ssnoversion-md.md)] 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 [!INCLUDE[ssSQL15](../../includes/sssql15-md.md)] SP2 CU2.  If that instance is upgraded to [!INCLUDE[ssSQL17](../../includes/sssql17-md.md)] 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.
+