Difference between average disk space and used space

Please see screenshot, can i reslove this issue?

If the used space on the piechart matches (or close to) what’s the OS reported, then you likely do not have issues related to a databases and/or filewalkers.
Otherwise you need to fix them first.
The difference between a real usage reported by the OS and the Average Disk Space Used This Month can be reduced only by the garbage collector. It’s initiated by satellites (they sends a Bloom Filters once or twice in a week), it will move the garbage to the trash, then after 7 days it will be removed permanently.

So do you suggest there’s around 2.6TB of uncollected garbage on that system?

It’s quietly possible. Need to have an updated stat on the dashboard in the piechart and then you may use the script

If your node received reports from all trusted satellites, it should give you a rough estimation how much uncollected garbage your node have.
Now I know, that this script is using only the latest stat, not the average, so it should be pretty accurate. The only requirements:

  1. Received usage reports from the trusted satellites
  2. Correct usage on the dashboard (on the piechart)