Sql-server – Backups in Availability Groups

No, this is not possible with Availability Groups. Any incurred delays are usually from things like network, secondary replica log block flush, etc. There's no way that I can think of to turn that knob and create more of a time gap.

It sounds like if you want to take this "delayed transactions" approach, Log Shipping may have actually been the best solution there. Or you could just deal with point-in-time restores in the (hopefully corner) case of data manipulation errors. Ideally that wouldn't be happening all that frequently, and you should already have point-in-time restores as part of your operational capability, so you wouldn't need to incur any other additional technology of processes to accomplish this.

If the secondary is up and running, when the log block is flushed to disk (either because it is full or a commit), the record gets pushed to the log writer on the primary and to the log scanner (log reader) process on the primary simultaneously. Then the log scanner communicates with the secondary and the secondary then pulls the transaction from the log scanner on the primary to the secondary and processes the log record. The primary log writer doesn't push transactions across, it just communicates with the secondary, it only does that to see if it is up so that it knows it doesn't have to mark the replica as NOT SYNCRONIZED.

When the secondary is not up, then the log writer cant communicate with the secondary so it marks it as NOT SYNCHRONIZED and stores the records in the tran log on the primary. If you look at sys.databases.log_reuse_wait_desc column it should show AVAILABILITY_REPLICA which means the primary is hanging on to all the records.

Once the secondary is up, it will communicate with the primary to request a log scan, it then processes the transactions and communicates with the primary using progress messages to indicate the hardened LSN, presumably the primary is then adjusting its MinLSN, which in turn means the records prior to MinLSN will get deleted as checkpoints happen and hence VLFs will get truncated releasing space when you do a log backup.

But yes short answer is, if your secondary is down you need as big a log file as you need for as long as it is down. Once it is backup and synched at some time you may need to remove the db from the always on group to shrink the log if it is humungous and you dont want it that big.