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How to reduce OC_Activitiy table
120M per day = 43 GB in a year 
380GB (95%) / 120M = 3116 = 8 years?
I think 120M a day or 95% is not correct.
Or it was higher in the last months and/or years.
I do not know what happens if you reduce the days.
Perhaps it is better to temporary deactivate CRON and execute it manually one time.
Also i do not know if cron delete old entries.