February 10, 2020 at 2:16 pm
Why differential backup is not supported on secondary replica in AG?
February 10, 2020 at 2:47 pm
I had a look at this myself, a few weeks ago - because log backups can be taken from any of the AG servers, yet differential backups can't. This is what I found:
Differential backups have to follow precisely from the last full backup and full backups can be taken at different points from different members of the AG. A differential backup on 'Instance1' would have the data required from the last full backup from 'Instance1' and no other. A full backup on 'Instance2' would have a different final LSN and so it would need its own corresponding differential backup. There’s no point in updating the primary with the last LSN of a differential backup on a replica, because it is meaningless. So Microsoft have just removed the ability to take differential backups from replicas.
Edited for typos.
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