Thursday, August 16, 2012

Apple Time Machine backups should not delete oldest, thin to monthly when space becomes short

The new version of the Time Machine backup capability in Mac OSX Mountain Lion has important improvements.

Most notably it transparently allows one to use multiple disks for backup. I use one backup disk at home and one at work. These allow me to be more confident that if one disk crashes I still have the other, and wherever I am working, I have an hourly backup to fall back on.

However here's a suggestion for the next improvement Apple could make: Currently the policy for maintaining backups is (quoting directly from the Time Machine control panel shown below):

  • "Hourly backups for the past 24 hours
  • Daily backups for the past month
  • Weekly backups for all previous months

The oldest backups are deleted when the disk becomes full."

My suggestion is this: When the disk becomes full, older backups should be thinned to monthly (up to three months ago) starting with the oldest. Weekly backups should always be retained for three months.

So if the disk becomes full, it should not delete the oldest backup, instead it should delete the second-oldest, and what was the third and fourth oldest, leave the what was the fifth oldest, and delete what was the sixth oldest, etc.

Why? Sometimes missing files can only be retrieved from deep history. With monthly backups far back in time, one has a pretty good chance of being able to retrieve such files. Weekly backups several years old are likely not needed.


In my experience I have retrieved lost work from Time Machine about 3 times a year. Not much. However, backups are all about insurance and peace of mind. Who knows when I will have a disk crash or do something catastrophic to my documents.

This post updates my earlier post on backups. Note that I no longer backup to Mobile Me since it has gone away. Some of my data is backed up on iCloud, although I personally feel it is critical to maintain one's own personal backup of cloud-stored data. I still back up using SuperDuper periodically in addition to Time Machine, although now I have two Time Machine disks, I will be updating my SuperDuper partitions less often.

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