Time Machine disc space

[BobSmith]

Originally Posted by John_G_Walter

What I am not really sure of, and maybe somebody here knows, is what exactly it does when it switches from hourly to daily to weekly? Does it somehow combine all of the hourly backups into one daily backup, or does it just keep the one of the hourlys as a new daily?

I don't know for certain either but my guess would be that it keeps the last hourly backup of the day as a daily backup and the last daily backup of the week as a weekly backup

I've used Retrospect for years to do incremental automated backup of my office computers. I use Chronosync to keep various volumes containing images backed up. Both of those have served me very, very, well for ages so I'm not dumping them. But I configured Time Machine on two systems that moved to Leopard a couple of months ago just to try it out. It's fantastic for being able to very quickly see how a given folder changed over a period of hours/days... or to retrieve an accidently deleted/altered file in seconds.

Bob Smith

[Paul Kuroda]

Hi John,
You mentioned that TM will erase the oldest files to make room for the new. That is totally what I thought it would do, but my experience shows that it doesn't and just fills up and requests for additional drives.

What am I missing? I couldn't find any preference setting to have it behave this way, and called tech support to no avail.

If this in fact did this...I would be very happy, since it would provide a safety net for the boot drive which is all I wanted. I archive my photos on externals.

If I cannot have it erase the oldest, then I'll try using Lightroom and have the files reside on externals, working on them there and prolonging TM backups with much smaller file changes/additions. Or....just do it the old fashion way and manually backup regularly.

[BobSmith]

I haven't filled a Time Machine disk yet so I'm not sure what will actually happen when I get there, but in Time Machine prefs, when you hit the options button it brings up a dialog with a check box to "warn when old backups are deleted". That seems to imply that old backups will be deleted. I have that box checked. I should find out soon what actually happens.

Bob Smith

[John_G_Walter]

Actually, from the TM help file:

"If your backup disk becomes full, Time Machine creates space for new backups by deleting old ones. To see a warning message when Time Machine deletes old backups, select this checkbox."

While this implies that you will never run out of space, elsewhere in the help file it states that if you run low on space, you should install another disk. However, I think that may refer to a scenario where the deletion of the oldest files are not really very old because of a small backup disk, and you may want to add space to get more depth in your backups. But, who knows??

I am running some tests right now to try to understand what exactly happens when the backups switch from hourly to daily, and from daily to weekly.

For example, if I create a file, have it backed up during the regular hourly TM run, then delete the file, the file remains in that hourly backup, but what happens to it when the hourly backups get replaced by a daily backup. Is the file lost, or is it combined with all of the hourly backups from that day? And what about different versions of the same file in different hourly backups? Does only the latest get saved into the daily backup? Or what? Does anybody know?

[John_G_Walter]

I've done a little testing, and now think I understand how Time Machine handles files.

As we all seem to know, TM runs a backup every hour, so if you add a new file, it will be included in the next hourly backup. If you make a change to that file in later in the day, the file will again be added to the next hourly backup, resulting in the consumption of two files worth of disk space.

At the beginning of a new day, TM treats the first hourly backup as its 'daily' backup, and begins deleting the hourly backups after 24 hours.

For example, the first version of that file that you added will get deleted when TM creates its hourly backup 24 hours after it was backed up, thus freeing the disk space that was devoted to it. The updated version of that file that you added later in the day, remained on the disk through the next morning's first daily backup, and it will now remain on the disk for 30 days worth of daily backups.

If, after those 30 days, the file remains on the disk, it will become part of the 'weekly' backup, and remain there.

When the backup disk is full, the oldest weekly backup(s) will be deleted to make room for the newer hourly backups. If that file remains on the disk, it will remain in the ongoing creation of hourly/daily/weekly backups. If it is deleted, then it will eventually disappear as time passes and the backups are deleted to create new space.

From this study, it seems that TM is designed to completely fill the backup disk, and actually is most efficient when doing so. Depending on the size of the backup disk, relative to the size of the data being backed up, will determine the depth of the backup in terms of versions of files.

I've made some mistakes already by moving files from one folder to another. Now, I have two 'versions' backed up, and they will remain there until my backup disk gets full, and the date of the file move works its way to the deletion point.

[Martin_Doudoroff]

Nice write-up about the experience of performing a complete system restore through Time Machine.

[Neil Cowley]

I'm glad to read of someone else with 1.5tb can use it but it told me that I'd need 1tb to backup my 250gb system drive. I don't really get that as I don't want to use time machine to backup my photo libraries and leave it to an 'automated' system that could start deleting things

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