Back In Time

Does BIT backup differential or incremental?

Asked by Removed by request

I would like to know if BIT is doing incremental backups or differential ones. So when BIT does incremental backups I would know that I better should not remove some snapshots...
Maybe this questions is not asked in the right way because rsync works in another way. I will describe it below on horuly backups:

hour00 - initial backup
hour01 - file A added
hour02 - file B added
hour03 - file C added
hour04 - file D added

Now I am deleting the snapshot "hour02" and restore everything with snapshot "hour04". Will "file B" restored or not?

I have read https://answers.launchpad.net/backintime/+question/70922 but did not understand it complete. For my case file B should be restored because the snapshots "hour03" and "hour04" still point on it with a hard link and the reference counter described in the other thread is 2.
But: When I delete the directory "hour02" from the snapshot directory ... where the hell is "file B"? Or does this "reference counter" thing only work when I am deleting the snapshot within BIT itself?

Thanks,
Eike

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Andreas Seifried
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Best Andreas Seifried (andreas-seifried) said : 		#1

Hi Eike,

"Now I am deleting the snapshot "hour02" and restore everything with snapshot "hour04". Will "file B" restored or not?"
--> Yes, it will be restored.

Please see http://de.wikipedia.org/wiki/Hardlink for a description of how hardlinks work.

File B will only be deleted if you delete all references to it. In you example: You need to delete hour02, 03 and 04.

You can check this by using the command 'ls -l'. The number shown between the access rights and the owner of the file indicates how many references to the file exist.

Regards,
Andreas

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Removed by request (removed3193549) said : 		#2

Hi Andreas,

thanks for the German Wikipedia link... should have searched for it on my own.

You described me that "deleting around" in the snapshots between will not affect files that are needed for other snapshots. Thanks for this!

I know this is the BIT FAQ but maybe you will help me to understand hard links better.

When I am deleting hour02 (on shell, in nautilus wherever) the directory and also the content is deleted, but because hour03 and hour04 are still linking on this inode it is still available.
So far, so good: but wouldn't this mean if I am deleting the backup with "shred" or "wipe" that these two programs are useless? Or are hard links a function that came to Linux with journaling file systems and due to the hard links shred and wipe became useless?
I know this straightaway goes off topic but maybe you have the time to answer my hard link questions

What happens when I update file B? Will it be taken to the snapshot like a new file or does BIT / rsync perform a binary diff and only copy the changed parts?

Sincerely,
Eike

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Removed by request (removed3193549) said : 		#3

Thanks Andreas Seifried, that solved my question.

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c_e_anderson@yahoo.com (c-e-anderson) said : 		#4

I'm hoping for further clarification about when BIT uses a copy of the target and when it uses a hard link. I've just started using BIT. I have mounted second hard drive with ext3 file system with the folders "/music" and "/backups" and I have specified BIT to include the folder /music and /backups as the backup folder.

I clicked the "Take Snapshot" icon from the BIT menu bar. The /backups folder is being used by BIT but it is creating copies of the files instead of hard links (I think so as ls -i indicated different inode numbers between to /music files and /backups files). Why is this so? I thought that BIT would use hard links to reduce the usage of hard disk space. I've also specified BIT to perform a 'weekly' scheduled backup (It would be nice to know when that's going to occur!). Will the next weekly backup be using hard links for unchanged files and will the hard links point to my manually created snapshot or will the hard links point to the original /music files? I run the risk of running out of disk space.

Thanks

Carl