What is a Synthetic full backup?

Can someone explain these?

I’ve set them on version 2 and can’t seem to see a difference in either time or size.

This is in the manual now:

8.5.4 Full image backup styles

You can select if full images should transfer all data or only data which changed from the last
incremental or full image backup (synthetic full image backup). A synthetic full backup will
transfer only changed blocks and store all blocks in the VHD/VHDZ file.
This option has no effect with the btrfs file system and the raw image file format because full
images will be disabled automatically there.

Cool. So basically, the full backup image size will be the same, but the time taken to complete the backup should be quicker because you’re only transferring changed bytes? If this is true, that’s great and should allow me to run full backups more often as my laptops currently don’t change much but take ages to transfer the full image data to the server.

If I’m NOT using BTRFS but instead ext4 (until BTRFS is stable) - can I still do synthetic full backups? Does UrBackup server recreate the VHD into one big image after a synthetic full backup?

Thanks in advance. I’m just getting used to 1.4 and it’s really great.


I would like to find out as well. Did you get synthetic backups working without btrfs?