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I just backed up the contents of my home partition onto my external hard drive using Back In Time. I browsed to the backed up contents in the external drive and under properties it showed me the size as 9.6 GB.

As I read that in next snapshots I create, Back In Time does not backup everything but creates hard links for older contents and saves newer contents, I wanted to test it. So I copied two small files into my home partition and ran 'Take Snapshot' again.

The operation completed within a minute - first it checked previous snapshot, assessed the changes, detected two new files and synced them. After this when I browsed to the backed up contents, I was surprised to see the newer and older backup taking up 9.6 GB each.

Isn't this a waste of hard drive space? Or did I interpret something wrongly?

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Most definitely a suitable question Chethan –  Allan Feb 19 '11 at 14:54

2 Answers 2

up vote 8 down vote accepted

It seems that checking properties under context menu is not reliable in such cases. When I used du command it showed right usage. I could get this suggestion from Launchpad's 'Back in Time's' Mr. Dan.

This is because backintime uses hard links, where each file is stored once, but makes it look like each copy takes up additional room. For more details please refer to https://answers.launchpad.net/backintime/+question/146072.

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My guess is that your external hard drive is formatted FAT32 or some other file system that does not support hard links. It also appears that if the partition type containing the file system is not a Linux (0x83) partition, the file system will not properly record the disk consumption of the hard links. If you create a Linux native partition on your external hard disk and format it with a native Linux file system such as ext4 it should work properly.

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Its a Western Digital My Passport Drive. I am using the default single NTFS partition it came with. –  Chethan S. Feb 19 '11 at 15:43
    
Also I tried deleting the older snapshot from the snapshots list. Still I can access all the files through the newer snapshot. Is this way of working recommended - Deleting the older snapshot once the newer one is created. I just can't stop thinking - the original snapshot took nearly 30 mins to create but the newer one was done by a minute or so! How did 9.6 GB of disk space fill in such a short while. –  Chethan S. Feb 19 '11 at 15:46
    
According to this link en.wikipedia.org/wiki/Hard_link, hard links are supported in NTFS. –  Chethan S. Feb 19 '11 at 15:52
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I have been looking at my back ups and when I back up to an external drive, even formatted ext4, I am seeing the same behaviour as you - each backup is allocating the full size of the backup as reported by the df command. When I back up to a subdir on my internal drive, each of the backups show the full size of the backup, but the df command only shows that a few Kbytes were consumed. I think this must be caused by the way the external drives are being auto-mounted or something. I'll keep investigating; this has me stumped! –  Dennis VanMeter Feb 19 '11 at 16:13
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I looked at my external hard drive and it has a partition type of NTFS (0x07) even though I have an ext4 file system on it. It does not report the disk consumption of the hard links correctly. I formatted my USB flash drive with a Linux partition with a ext4 file system and it now reports the disk consumption correctly. This seems to work around the problem, at least for me, since I don't need to plug my backup disk into a Windows computer. I'll look around a little more to see if there is some way around this problem for NTFS. –  Dennis VanMeter Feb 19 '11 at 16:58

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