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Hi there!
From the notes, I understand that you are getting an error in Aurora logs that says: “Bytes_behind_primary (12345) is smaller than aurora_binlog_io_cache_size (12345) by 12345.” However, this is not an issue even though it is written in the error logs.
Please note that the dump thread metrics are printed to the database log every 60 seconds with the prefix [Dump thread metrics]. The metrics include information for each binlog replica such as Secondary_id, Secondary_uuid, binlog file name, and the position that each replica is reading. The metrics also include Bytes_behind_primary representing the distance in bytes between replication source and replica. This metric measures the lag of the replica I/O thread. That figure is different from the lag of the replica SQL applier thread, which is represented by the seconds_behind_master metric on the binlog replica. You can determine whether binlog replicas are catching up to the source or falling behind by checking whether the distance decreases or increases.
The error log you got is just a message that checks how far the replication distance of the binary log is between replication source and replica in the point of view of Bytes. This is not an instant system error so you don't have to worry about it.
Also, this message is shown in Aurora MySQL 2.10.x and higher for every 60 seconds. You may check document [1] on optimizing binary log replication. Unfortunately, there is no way to exclude this log in error logs currently.
I hope you find the above information helpful.
Have a great day ahead!
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