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Subscribing multiple SQL Server databases across instances to the same SQL Server service plan, to create copies of multiple databases in parallel, is supported. There is a multithreaded approach in the AppSync host plug-in that enables application-consistent backups of multiple databases in parallel. It provides a performance boost. AppSync divides the subscribed databases into efficiency groups. By default, each group must not have more than 35 databases per instance on a host. This grouping avoids flooding the host plug-in with requests and ensures load balancing during the copy creation process.
For two instances on the same host, a group can have a maximum of 70 databases altogether, with a maximum of 35 databases per instance. While the first group is being protected, the other groups that belong to the same host wait until the protection of the former group are completed. In an instance, the number of databases that are protected together is restricted by setting the Maximum number of SQL databases option in Advanced Plan Settings under the Define the Copy step of the SQL Server service plan.