Backup file size increasing over time

[focus]

Hey guys!

I've noticed strange behavior of the .vbk file.
It slowly grows each time VEB makes incremental backup merge, even if increment has info about deletions only.

I've spotted it for my full PC backup first (it slowly growed from 740 GB up to the 769 GB in ~1 month).
And I decided to make a small experiment and configure a backup with such settings:
- for folder with one small file (14kb)
- keep restore point for the last 1 day
- disabled all other settings: http://i.imgur.com/uBBZD2g.png

And started experiment.
I've made a backup for the first time, resulting backup file:
30982kb
Next I've duplicated that 14kb file multiple times, made few incremental backups in a row (each about 30mb in size) changed date to the day forward (to force the incremental merge) and made other backup, and I saw after merge .vbk increased in size.
I've removed duplicates, moved one day forward and made one another incremental backup.
And .vbk increased in size once again.

I've repeated it few times and here is how .vbk file size changed:
30982
31059
31078
31433
31490
31548
31608
31667

So, after few repeated cycles I have:

Folder with 1 small 14kb file as a backup target, just like at the beginning of the experiment.
And backup file for that folder (having only that 14kb file inside) increased in size from 30982 to 31667kb after few cycles of merges.

I can repeat it, so I'd be glad to help you catch this issue up, or it's an expected behaviour?

[Mike Resseler]

Dmitriy,

Every incremental will have some information. The 30 MB change you see with almost no change is rather normal. The bigger change, the less you will notice this. It looks rather normal to me to be honest

Mike

[focus]

Hey, Mike!

I'm totally fine with 30 megs increments actually.
But I don't think the actual final backup file should grow in size after few increments if it has same content as it had initially.
Literally I have the very first backup, 30982kb in size and it has one 14kb file inside.
And I have second backup, after few incremental backups merges cycles, with same 14 kb file inside, but with bigger self size (31667kb in the test case).
This is not looking fine...

[Dima P.]

Hi Dmitriy,
Even though when increment is merged into full and some data blocks should be erased we simply cannot do that doe the architecture reasons of the backup file standard. So instead of actual erase, “dirty” blocks are marked to be reused during next merge. I believe that’s why you see a full back up being slightly changed in size from time to time.

[focus]

Hey, Dmitry!

Thanks for your answer, now I see how it works.

[Dima P.]

You are welcome – I am glad to hear we are on the same page now.