A few questions about backup storage mechanisms

Server: UrBackup 2.4.12, debian 10.3 64bit / Linux 4.19.0-8-amd64);
Storage file system: BTRFS.

I did not find clear answers to the following questions:

File backups:

  • There is a 3GB file, create a backup, then change the data in this file to 100MB, but the total file size is still 3GB. Then create a backup again. How much disk space will the new backup take (it will increase only by 100Mb of changed data or by the full size of the whole file, i.e. by 3GB)?

  • If I delete the intermediate backups, then the data integrity is not violated? Thus, if I restore one of the last copies, then the integrity of the data will not be violated? I would like to understand in more detail how the copy storage mechanism works, maybe there is a link to the information.

  • If I delete all full backups, the integrity of the data will be violated? I tested, deleted all full copies, performed a cleanup, then restored data from several backups, everything recovered successfully. How is this possible?

  • Why do I need to make full backups of file backups? As I understand it for reinsurance (be more safety)?

Image backups:

  • Answered (below) - What is the difference between incremental backup and full image backup? In case with file backup the server downloads only new files in incremental, then in the case of images this mechanism cannot work.

  • Answered (below) - There is a 3GB file, create an image backup, then copy this file 3 more times. Then create an image backup again. How much disk space will the new backup take?

  • The same question that I wrote earlier is but only about image backups. If I delete the intermediate backups, then the data integrity is not violated? Thus, if I restore one of the last copies, then the integrity of the data will not be violated? I would like to understand in more detail how the copy storage mechanism works, maybe there is a link to the information.

  • The same question that I wrote earlier is but only about image backups. If you delete all full backups, the integrity of the data will be violated? I tested, deleted all full copies, performed a cleanup, then restored data from several backups, everything recovered successfully. How is this possible?

I would be grateful if you answer each item.

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I found answers a few qestons:
Image backups:

  • What is the difference between incremental backup and full image backup? In case with file backup the server downloads only new files in incremental, then in the case of images this mechanism cannot work.

If the storage file system is btrfs and the raw image file format then full images will be disabled. All image backups always incremental! This is due to the features of the BTRFS file system.
More information about it:

  • There is a 3GB file, create an image backup, then copy this file 3 more times. Then create an image backup again. How much disk space will the new backup take?

We copied 3 times a 3 GB file. These files took up additional 9GB of disk space, so they used blocks on the disk for 9GB of space.
The new image backup take 9GB at storage(without compression), because incremental image backup copy changed or new blocks (512 kbyte chunks), so UrBackup will copy this new blocks which size is 9GB.
I checked this and the numbers match (df -h 9Gb more).

If anyone knows answers to another questions, please, answer!