Time Machine deletions when the disk is full

I would like some clarifications regarding how Time Machine goes about deleting files when the external drive is full. At the outset if one has backed up everything including system files, does this mean that they will go first, the actual operating system, or does it delete the earliest versions of currently existing files? Can anyone enlighten me please? Thanks. I've just been rebuilding my Mac and am wondering whether to backup just documents or the whole system.

Time Machine basically, once you Ext is full, it will start deleting backup's from the very 1st time you did it. Then the next and so on. So if you did your 1st backup 1/3/2009 @ 1015am and then every hour after that it will delete 1/3/2009 @ 1015am, then 1/3/2009 @ 1115am, then 1/3/2009 @ 1215pm and so on.

Does that make sense ??

TM

(Edit - deleted this line since I didn't know what I was talking about.)

It deletes the oldest backups first - by date. Everything in the oldest backup.

If you have a habit, as some do, myself included - of moving a lot of files on and off your machine that don't need to be backed up to TM, exclude the folders those type of files go into from the backup.

Ex. I exclude the downloads folder. I download and run a ton of apps without actually installing to test them out and having all of them backed up would be a waste of space.

Thanks guys. That helps.

bobtomay said:

TM does not back up all the system files. It does back up your preference files.

Click to expand...

Bob, I'm not sure I'm understanding you, but as I read you, this is not correct.

Time Machine backs up EVERYTHING. Applications, system files, preferences, user data, photos, music -- EVERYTHING that is on the hard drive, including invisible files. Every last byte of it (file-by-file not sector-by-sector, but still).

It then adds an "iteration" of what's changed. So if your original state of the hard drive can be expressed as "A", and whatever has changed since then can be expressed as B, then the 2nd backup is A+B1, 3rd backup is A+B2 and so forth.

Time Machine doesn't actually *ever* delete the original backup (the "A" state). It deletes the oldest "B" states. The only time the "A" state gets erased/replaced is when a new "A" state is created (by changing the hard drive substantially enough like by a system update). This is why Time Machine doesn't "recognise" your hard drive when you upgrade from Leopard to Snow Leopard and you have to do a new full backup.

Time Machine conserves space by folding the hourly updates into a daily update after 24 hours. After 30 days, it folds the oldest daily backups into a weekly backup. These weekly backups are what start getting deleted (oldest first) after the hard drive gets close to full. The A state is left untouched.

Hopefully that clarifies things a bit.

PS. Your suggestion to exclude certain "busy" folders such as the download folder from TM is a particularly good one.

As well as excluding your downloads folder, you can also free up more space by deleting unwanted files, for instance - I regularly delete all back-ups of my Virtual Box windows disk, deleting all old copies and safe in the knowledge the latest copy will get backed up on the next TM back-up.

Thanks for the info. Given my external drive is not large and I can't yet run to a new one, maybe I'd be best just backing up documents etc and also doing a regular save to something like Carbon Copy Cloner.

I stand corrected - again - what else is new.

Now it seems to me even more silly that you have to install the OS before you can restore.