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Log backups are important for point in time recovery and to automatically truncate the log file (in full recovery model) to make space for log reuse.

But when the secondary is unreachable either due to network issue or post failover, then I'm learning that log keep growing (because it prevents truncation: https://www.sqlshack.com/sql-server-transaction-log-growth-monitoring-and-management/).

In this scenario, log backup won't truncate the logs, so does that mean the log backups that occur behave like copy only log backups? That is - log backups won't truncate the logs (so each subsequent log backup size keeps growing) and yet it also allow for point in time recovery.

Is that a correct understanding on the log backup behaviour?

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[...] does that mean the log backups that occur behave like copy only log backups?

No, they are normal log backups. Everything behaves as it should, since the log_reuse_wait_desc in sys.databases would show AVAILABILITY_REPLICA and not LOG_BACKUP then it makes sense that this would not truncate.

[...] log backups won't truncate the logs (so each subsequent log backup size keeps growing) [...]

Whatever log is generated between each backup is what will be backed up, there is no change. Also please note that log backups are just one mechanism of checking if log can be reused.

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