Aging algorithm...

Question

Hey there..
Can anyone give any insights on the aging algorithm that TM uses? I mean this application is going to back up on a regular schedule until all 500GB on the drive is consumed.. but then what? How does it decided what to "get rid" of in order to maintain integrity of the backup data over time?

Answer 1

TM maintains backups as follows
The initial backup is the first hourly backup.
1. Performs hourly snapshots for 24 hours (can be a few more than 24 at times)
2. The first backup that runs after midnight will take the last hourly backup done prior to midnight and save that as a daily backup.
3. The daily backups are collected up for 30 days (or one month)
4. After a month and one week or partial week all daily backups for Mon thru Sat are discarded and the Sun daily is kept as a weekly backup.
5. As each week passes the action in 4) above is performed.
Soooooo, say after 6 months you will have 5 months of weekly backups (constituting some 20 t0 22 weekly backups) and 30 daily backups and a few hourly backups.
When the disk fills TM will try and remove some of the old weekly backups to make room. It's unclear to me just how far TM will go to make more room for the current backups. However, if the current backups are relatively small I believe TM will simply remove a few of the oldest weekly backups until it has regained sufficient disk space to complete the current hourly backup.
TM will always keep important OS related files so that a full OS restore can be done. Typically these OS related files are captured during the initial backup. However, more OS related files can be captured immediately TM backup executes after you've done an OS upgrade such as 10.5.1, 10.5.2, 10.5.3, etc.
Hope that clarifies things for you.
Good luck.