What really happens when Time Machine is "cleaning up"? [duplicate]

When TimeMachine runs a backup it goes through several phases. There is the "prepare for backup", "backup" and "clean up" phases. There might be more.

What happens during the "clean up" phase?

Solution 1:

When Time Machine goes through the "clean up phase" it's doing "house cleaning" which will consist of activities like:

• delete old/expired backups (i.e. past X number of days)
• removes old(er) backups if there is insufficient space
• trimming incomplete backups

This process may take a while, especially with many small files, as it must delete every file and folder while ensuring the backups stay consistent. For each backup to delete, it must delete all changed or removed files (compared to the kept backups) as well as remove references to identical files.