Aging algorithm...

When the drive is full Time Machine starts deleting the oldest backup first keeping all the files necessary for a full restore of the system. If a file is on your computer for less than a day and it isn't in the hourly backup done at midnight or the first backup after it will be deleted in the process of Time Machine thinning the hourly backups to dailies.

It is not yet absolutely clear what Time Machine keeps in the daily backups when they are thinned to weeklies. To be on the safe side it's probably best to keep files on your drive for eight days before trashing them if you want Time Machine to retain backups of those files until it runs out of disk space.

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.

BarryXSharp wrote:

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.
BarryXSharp wrote:

While I don't think any of us really know what goes on under the hood, I don't think it's as simple as removing the oldest weekly backup. Each backup is a collection of files that are saved for the first time or links to files that were previously backed up. So if the oldest backup includes files that newer backups have links to, I'd be surprised if the real file is deleted.

One possibility is that, within the oldest backup, the number of references to each "file" is checked, and if only one reference exists, then that file is deleted, otherwise only the link to the file is removed.

But it would be interesting to read about the algorithm.

ElPapa,

Actually, we do know....

Time Machine will begin deleting the oldest backups. Period. It won't "thin" the files in specific backups, it won't skip backups, it will simply begin deleting the very oldest backups, oldest to newer, until it has freed up enough space for any new ones.

Chances are, this means that at least several of the oldest backups will be deleted, given the multi-linked nature of the Time Machine "snapshots." In any given snapshot, only a relatively small number of files will be unique, so many of these snapshots must be deleted before any real disk space is recovered. At some point, all of the snapshots that are comprised of a given OS version (10.5.0, for example) will have been deleted; it is most likely at this point that some substantial disk space will become available.

Of course, those that find themselves in this situation the soonest will be those that tend to have a large amount of additional or changing files, a lot of "data churning," as it is called. Fortunately, it will be the backups made under these conditions that will be the easiest to "thin" by deleting old backups.

Rest assured, however, that at no time will one not have what is required for a complete system restore. The latest backup always links every file necessary for a complete restore.


Scott