"Daily" archive creation bug in Time Machine?

[michaelsanford]

I was curious, when looking through my Time Machine backups, how exactly it decides what goes into a "daily" backup. After some searching, I found this

At midnight, or as soon thereafter as your computer is on and the backup drive available, the most recent hourly backup will be saved as a daily backup, and earlier hourly backups will be deleted to save space. Each daily backup is maintained for a month. After a month, Time Machine converts one of those daily backups into a weekly backup that it preserves indefinitely (or until your backup drive becomes full).

If this really is the behaviour of Time Machine, I think we've got a bug.

What if I've created a file at 12 h 00, accidentally deleted it at 16 h 00 but need it the next day? Shouldn't Time Machine apply the same clever diffing system it uses to choose which files to back up, to the whole day's backup and keep a backup of the whole day's activity?

(Sorry if this should go in Open Letter, but I felt it would better live here in case someone can contradict me, which would be great.)

 

[ElDiabloConCaca]

I do believe the daily backup that is turned into a weekly backup is the latest, most recent daily backup.

So, if you made a big PhotoShop file at 10am, deleted it at 2pm, then used your computer until 10pm and then turned it off shortly after, then your weekly backup will be the daily backup taken at 10pm which will not include the PhotoShop file.

Completely normal operation, it seems to me.

 

[michaelsanford]

I understand your example. It is normal operation, insofar as far as Time Machine has been designed that way. Perhaps I should have called it a 'design flaw' instead of a bug. Or perhaps misleading (though not intentionally). Prone to misunderstanding?

Time Machine is billed as an hourly backup system. I assumed (obviously incorrectly) that the shortest interval I had to wait for something to be found in my Time Machine backup was, therefore, one hour.

I mean that very precisely: that as long as something had been backed up at least once, it would be in my backup, and that the hourly backups would be aggregated somehow into a daily backup, and not that the last daily backup would be taken as an accurate snapshot of the day's work (as my example shows, it isn't necessarily).

It is also not my intention to compare this to other backup systems that take only daily backups: they obviuosly all are obviously 'flawed' in the same way as they would take only one backup, and never be aware of the file in the first place. The point is that Time Machine IS aware of every hour's work, so why not aggregate?

EDIT: ElDiablo I was referring to hourly>daily backups, not weekly, though the issue would seem to exist for that as well.

 

[Mikuro]

I've also heard that Time Machine will only store deleted files for 30 days. So it seems that Time Machine doesn't care about deleted files so much as old versions of existing files. I'm not sure how I feel about this, to be honest.

So basically, you shouldn't count on Time Machine to save files you delete. Ever.

 

[jbarley]

I do believe the daily backup that is turned into a weekly backup is the latest, most recent daily backup.

So, if you made a big PhotoShop file at 10am, deleted it at 2pm, then used your computer until 10pm and then turned it off shortly after, then your weekly backup will be the daily backup taken at 10pm which will not include the PhotoShop file.

Completely normal operation, it seems to me.

Correct me if I've misinterpreted your example,
Here's what I've experienced while using Time Machine.

That file should still be there, in that timeframe TM would have completed at least 2 maybe 3 backup cycles, once backed up it will stay in TM until you fire up TM, select said file in the TM Finder window, then from the drop-down menu select "Delete all copies of 'filename'".
Everything Time Machine backs up stays until it gets purged when the disk get full, hence the complaint some have with TM needlessly filling the disk.

 

[michaelsanford]

jbarley, that's precisely what I thought happened, but it seems that it's not. Contrast my example with that of ElDiablo's: I thought that a file that was backed up at least once, say, at 13 h 00, would appear in that day's daily backup. Apprently, it's only the system snapshot taken at 24 h 00 that day that becomes the 'daily' backup, and the other hourlies are erased.

This means that if I created a file and deleted it within the same 'day' as far as Time Machine is concerned, it won't be in the daily backup for that day. This has been my experience. I created a folder with my router configs on my desktop at around 13 h 00. I used it until around 17 h 00 and then deleted it. That day, I realised I had made an error and retrieved it from Time Machine, applied the binaries, and deleted it again. But the NEXT day, looking for it, it did not appear in the previous day's daily backup, because, by midnight the previous day, it was already trashed.

At least I'm not the only one to have understood it that way