AWS - Backtracking an Aurora DB

Question

According to this link:

The actual backtrack window is the actual amount of time you can backtrack your DB cluster, which can be smaller than the target backtrack window. The actual backtrack window is based on your workload and the storage available for storing information about database changes, called change records.

To modelize the statement above, I make this graphic:

How can actual backtrack be smaller than target backtrack? I think actual backtrack should be larger than target backtrack, so that it can have sufficient change records for backtracking?

In my graphic, what if I want to backtrack to 24 hours earlier, which is outside of actual backtrack window, so it does not contain change record?

Is this a typo in this document? Please explain if I'm wrong.

Thanks,

Answer 1

The target is what Aurora attempts to keep available at all times. The actual amount of history available is necessarily the same or less.

You can think of your target backtrack window as the goal for the maximum amount of time you want to be able to backtrack your DB cluster. In most cases, you can backtrack the maximum amount of time that you specified. However, in some cases, the DB cluster can't store enough change records to backtrack the maximum amount of time, and your actual backtrack window is smaller than your target. Typically, the actual backtrack window is smaller than the target when you have extremely heavy workload on your DB cluster. When your actual backtrack window is smaller than your target, we send you a notification.

https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/AuroraMySQL.Managing.Backtrack.html