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Old Feb 19th, 2012, 01:06 PM   #1
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Back Up Question w/ Time Machine

So a whilst back, I bought a new ext for my main backup. and plugged it etc... everything was fine. some days later, the back up refused to work and somehow I was able to create a new back up folder (how this took place I still don't recall). So now I have 2 back up folders: The old one (A) and the new one being currently used daily (B). But now my question is this:

is the old back (A) up now just taking up space?? Can I delete it?? It's been SOME weeks now since it's not been used as you'll be able to tell from the screen grabs.

Old back up (A)



Current back up (B)



As you can tell, the dates on the old one are long behind, and the current one is all well.

Can I DEL the old folder??

+

Another quick q?

When I did a APPLE i on the old folder, as I wanted to get an idea of the size, I got this:



2.23TB???

How is that possible from a 1.5 TB HD?? Is cold fusion back on the table??

Anyhow, I feel like I may have asked this before, but certainly no screen grabs. Sorry about the repost if so.

THX for input.
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Old Feb 19th, 2012, 01:42 PM   #2
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Yes you can delete the old backups. If Time Machine is using a different folder/drive then it shouldnt make any difference.

And 2.23 TB from 1.5 TB is possible because it backs up new copies of files that change, and keeps files that you may have deleted.
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Old Feb 19th, 2012, 09:14 PM   #3
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To be more precise about the 2.3TB on a 1.5TB space... Time Machine uses hard links on the file system. A link is just like a shortcut at the file-system level, where 2 files can actually point to the same location on the disk. This makes it easy to have a "full" back up in each of the dated folders without creating unnecessary duplicates of the files.

Let's say you have a file in your Home called "Letter to friend". Time Machine backs it up the first time and creates a file in the 2012-02-19-2020292 (those last numbers are made up) folder called "Letter to friend".

Keep in mind a file name on a disk is actually a label with a pointer, and said pointer points to a physical location on the disk where the file starts.

Next day, if the file hasn't changed, TM will once again create a file in 2012-02-20-1202826 folder called "Letter to friend", except instead of really creating a completely new file, it creates as before a label with a pointer, and that pointer will point to exactly the SAME location as the previous days's "Letter to friend" file.

So you have "2012-02-19-2020292/Letter to friend" -> 3872531517 (file's location on disk)
and you have "2012-02-20-1202826/Letter to friend" -> 3872531517 (same location on disk)

Of course if you ask finder how big both of those files are, it will tell you both are (for example) 78kB, for a total of 156kB, even though in reality there are only 78kB used on disk because both files are the same. This is how the Finder thinks there are 2.3TB of files on your 1.5TB disk.

The next day you modify the file, and when TM backs it up it will actually create a NEW file, starting at a new location on disk, that includes the new, modified content of "Letter to friend". Now there are 234kB of files on the disk, even though the actual real space used is only 156kB.



Now here's the interesting part, when you delete a file, what actually happens is: the label and pointer are deleted. Moreover, as long as there are pointers to that specific location on disk, the file is not really deleted. So if you delete 2012-02-19-2020292/Letter to friend, that file will STILL exist physically and still be accessible because "2012-02-20-1202826/Letter to friend" still points to the same location.

Only when all pointers are gone does the system say "ok these sectors on disk are now free and can be written over". As long as there are pointers to the location, the system does NOT write over these locations as they are used by files.

This is why files can often be undeleted, because even though the label/pointer are gone, the actual physical locations on the hard drive may still contain the old data because they have not been overwritten by new content yet.


Anyways this is basically how TM works and how it can get away with only backing up the changed files on your system while still showing you ALL the files of your system in each of the backup date folders, because almost everything is basically a hard link. This is also how it manages to easily delete old backups when the drive gets full, because again it's all hard links so it doesn't really matter if the oldest backups are deleted, as the files will still be on disk as long as there are pointers to their physical locations.

Whew.
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