Back In Time

Why are Back In Time snapshots so large?

Asked by Chethan S.

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?

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?

Regarding file system used, home folder is ext4 and external hard drive is formatted in NTFS. I had the same experience when I tried backing up the contents of other NTFS partitions in an internal drive onto a NTFS partition in my another internal hard drive.

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Chethan S.
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Dan (danleweb) said : 		#1

As I understand you check the size of each snapshot folder and both have ~9.6 GB, so you suppose that both snapshots use ~19.2 GB. Please check the size of the folder containing both snapshots.

The most reliable method to calculate to size of a folder if using "du" command.

Regards,
Dan

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Chethan S. (chethan) said : 		#2

I just ran the command 'du -chS 1' where 1 is the folder in which the home's backup resides. It showed me '9.8G total'. As I had deleted the first backup of my home folder created yesterday, I again took a snapshot today after copying a small video file. Now when I checked using the same command it shows '9.9G total'. If I check the size of two snapshot folders inside '1' using properties in context menu, they show 9.8 GB each. If I check the properties of '1' it is just 9.8 GB. Therefore, I think 'du' should be the right command to use.

But I am puzzled by one thing - though I deleted the first snapshot created yesterday how did 9.8 GB data remain in 2nd snapshot. Was there any problem yesterday? Now can I rely upon the snapshots what I have or do I need to delete two snapshots and create one afresh? Please guide me.

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Dan (danleweb) said : 		#3

Just a quick info about hard-links (you can find more on google): let's say you have a file of 100Mb (1.bin). The file system store the file name somewhere and the data in another. When you create hard-link (2.bin) the file system will store the new name but it will point to the same data. The data has a reference counter. This counter contains the number of hard-link. If you delete a file, the system will remove the name and it will decrease the reference counter of the data. If there are not more references (ref counter = 0) the data will be removed.

Regards,
Dan

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Chethan S. (chethan) said : 		#4

From your info I understand that my backup is safe due to the existence of hard links in the 2nd snapshot. Also since it shows the right size and I can physically access the files through the existing backup even under my dual-boot windows installation I understand that everything is safe.