Performance improvements include:
- When old backup dir count reaches maximum use last one as new destination dir (mv old new) - no need for deletion, rsync will sync its content. If more than one old backup directories need to be removed (config changed) then the rest is deleted.
- Use rsync --delete with empty directory for removal instead of rm -rf, since rsync deletion can be faster than rm.
- Do deletion in background while continuing with creating new backup. Before deletion and before creating new backup directories for deletion are moved into hidden trash directory so that they won't be listed in following ccollect steps.