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Time Machine Backup Retention

I was hoping someone could help me better understand the rules Time Machine (TM) uses when keeping or deleting old backups.

I know that TM keeps hourly backups for the past day, daily for the past month, and weekly after that (until the disk is full).  I am looking for a bit more detail on which backups are kept and which are discarded.  Some specific quandaries are listed below, but if you know anything more, I would love to hear it.

• Does "hourly backups for the past 24 hours" mean it keeps one backup each our from the 24 hour period starting "now", or does it keep 24 of the previous hourly backups, or just hourly backups from the last calendar day?
• At some point 23 of the 24 hourly backups per day are deleted when it goes from "hourly" to "daily for the past month."  Does this happen at midnight? Which hourly backups are kept?
  • I have seen people claiming that the first hourly backup of a calendar day is kept as the daily backup, is this true?
• If only one backup per day is kept long-term, what happens to data that only shows up for part of a day? For example, I receive a file from a coworker or family member and save it to my Documents.  I upload the file to my work/home server later the same day and remove it from my Documents folder.  If the file was not present when the daily backup was performed, then is it unrecoverable after 24 hours?

In case it matters to anyone, I am running TM on my 2015 MacBook Pro 15" with macOS 10.12.6 and my backup drive is the LaCie Rugged USB-C drive sold by Apple (contains a Seagate ST2000LM007 HDD)

Thanks,

Eric

"Hourly" backups are retained for 24 hours. The oldest of the 24 is retired upon the next "hourly" backup. The first backup of the day – whenever that occurs – is retained as that day's "daily" backup though.

• I have seen people claiming that the first hourly backup of a calendar day is kept as the daily backup, is this true?

Yes.

• If only one backup per day is kept long-term, what happens to data that only shows up for part of a day? For example, I receive a file from a coworker or family member and save it to my Documents.  I upload the file to my work/home server later the same day and remove it from my Documents folder.  If the file was not present when the daily backup was performed, then is it unrecoverable after 24 hours?

That's an important question. If you delete something from the source volume, it becomes a candidate for deletion upon the very next backup. In other words, unless that backup just happened to be designated as that day's "daily" backup, it will be deleted.

最後更新：2017-08-22 07:20:15

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