How does Automatic Seeding work - Always On Availability Group

Question

I found only these two statements from BOL regarding this Initial Data Synchronization method:

Link 1

Automatic seeding uses the log stream transport to stream the backup using VDI to the secondary replica for each database of the availability group using the configured endpoints.

Link 2

During automatic seeding, SQL Server performs a backup over the network for initialization.

Looking into SQL Server Error logs of both primary and secondary replicas, these two logs caught my attention:

On primary replica

On secondary replica

My question is related to the working nature of automatic seeding, namely where does taken backup is stored before restoring on the secondary replica?

Answer 1

As you noted, the docs refer to "log stream transport" when performing automatic seeding. Similar to the way that you can stream movies to your device without downloading a specific file, automatic seeding streams data to the secondary without storing an actual backup file.

While the technical implementation details aren't exactly this, it's essentially taking a backup, but rather than sending it to a storage device or file share, it sends it to the secondary server. Rather than writing a file, it immediately begins to restore to the secondary server. The pseudo-backup is buffered and restored in a single step, without a literal backup being written to file.