Back In Time

How does BiT actually work?

Asked by Martin Fisher

I have been using BiT for a few weeks now to backup my system, and I am very happy with it- of the many backup tools I have tried this suits my needs closest. However, I have one nagging question: Howe does it actually work? I believe it uses links so that not every file needs to be actually physically present in each snapshot (although I don't understand how this concept of a link works).

For example: Backing up every hour, if a file me.pdf is created at 13.55, the 14.00 snapshot will contain it. The 15.00 will also contain it. If at 15.15 the 14.00 snapshot (only) is deleted, will me.pdf be in the 15.00 snapshot?

With thanks, and apologies for my ignoance!

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Dan (danleweb) said : 		#1

It the file me.pdf it is not modified between 14.00 and 15.00, then "take snapshot" process will make a hard-link from 14.00 to 15.00. Hard-links are like different names for the same content. Each content has a reference counter. When you remove a file it will remove the name and will decrease the content's reference counter. If the reference counter is 0 the content will be removed too. In your case you will have me.pdf names in 14.00 and 15.00 and one content with reference counter: 2.

For more information you can read this: http://en.wikipedia.org/wiki/Hard_link

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Martin Fisher (yusuf-martin) said : 		#2

Many thanks, and I have read the wikipedia entry. This is how I understand it:

Returning to my original example, if I delete the 14.00 snapshot but not the 15.00 snapshot then the physical file actually still exists because the reference counter will be 1 (whereas if I also delete the 15.00 snapshot the reference counter will become 0 and the file will no longer exist or, at least, the operating system will reclaim the space). Is this a correct understanding?

This leads me to two further questions:

1) The point of my original question was to determine how best to make a backup of my Home directory if I am going to do a fresh install of Kubuntu. When doing this I don't want to restore every file that has a reference counter >0 (as is on BackupInTime, I think [!]) - I simply want to make a clean install and then restore my Home directory in the same state as it was prior to the fresh install. For doing this, would it therefore be best to use your other utility, YADsync?

2) When making a fresh install as in (1), how can I ensure that when I reinstall BackupInTime it still recognizes and uses all the snapshots that already exist on my external USB drive?

With grateful thanks if you have the time to answer this ignorant questions!

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Best Dan (danleweb) said : 		#3

1. I don't think I'm the best person to suggest you how to backup your home directory. I can suggest you that in the future to create a partition for root and one for home. This way you will not have this problem anymore.
You can use BackInTime or YADSync or something else (just a simple copy or a tar will do) but then just don't restore everything at once. Restore your work and then step by step items you think you may need.

2. Just set snapshots path.

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Martin Fisher (yusuf-martin) said : 		#4

Thanks Dan, that solved my question.