I have had backup config created 2 years ago and now it is over 100gb.
backup retention was set to “Keep all backups”
Is it safe / Is it going to work if i edit config now and change retention to “Delete backups that are older than” and specify time (i am thinking 2 months)?
What version are you currently using? I recall a bug a while back that could cause Duplicati to fail if retention was adjusted and it resulted in too many backup versions needing to be deleted, but that bug was fixed a while ago.
Edit: it was fixed in 2.0.4.26_canary. If you are running the latest beta 2.0.5.1 you should be fine.
Deletes (like backups) can take awhile for database and destination work. Use small batches if you prefer. Processing involves a lot of database updating, and then compact will clean up now-wasted space awhile. Local destination will run pretty fast. Cloud destination could download and upload awhile for the compact.