Incremental backup file larger than full backup file

[evandeuren]

We use 2 backup jobs, forever incremental. Strange thing we are facing, on the main repository we have 2 full backup .vbk files (1.2 TB and 4.8 TB) and several incremental .vib files (between 200 and 400 GB).
We have a copy job to second off site repository. This is one copy job handling both source jobs.
I would expect on the offsite repository to see one large .vbk and several .vib files, like on the other repository.
But I see one .vbk (892 GB) and several .vib files, where the largest .vib file is 5.1 TB.

Recently we were having an issue with slow merges because of a problem with ReFS on our repository.
This was solved by starting new backup chains which now use the fast clone merge functionality.
With the previous backup chains, files sized were as expected.

Is this expected behavior or are we facing some kind of issue?
Any help is much appreciated, thanks.

[DGrinev]

Hey evandeuren and welcome to the community!

That happened because backup copy job doesn't make a copy of backup files, but creates a new independent chain of restore points based on the most recent VMs state in the source backup chain.
Thus, during the incremental run the copy job delivered VMs data contained in the source full backup file. Also, .vbk file can be smaller than .vib if the job didn't deliver all the data before the interval end and continued during the next run.
Don't hesitate to ask additional questions for the sake of clarification. Thanks!

[evandeuren]

OK, and what would happen then if I run an active full on the copy job? It would create a large .vbk file if I'm correct, and when the limit of restore points is reached, it would delete both the old small .vbk and the old large .vib?
thank you for your explanation and time!

[DGrinev]

That's correct, the previous backup chain will be removed by retention. Thanks!

[BGA-Robert]

Are your backup copy jobs set to compact on a regular basis? Otherwise they will tend to forever grow.

[evandeuren]

Yes, we do have the compact option set to run every month, forgot to mention that. Tried to run it last week, but it created a new full backup file out of the existing vbk, so still not the size I was expecting.
I ran active full last night, and now I have a 6 TB vbk, and will wait untill the old files will be deleted by retention.
Thanks!