- I like to have a real time backup.- online erase function is not acceptable
- looking for online service
I think the combination of requirements somewhat paints you into a corner. For online erase I am assuming that someone wants you and/or your data forgotten.
An option that meets your real time and immutable requirement would be multiple contracts with data-vaulting companies that will legally protect N+2 copies of your data spread across multiple regions. These companies charge a few left arms and a couple right legs every year.
A more cost effective solution would be to install big servers with a few of your friends and family members that have high speed internet such as fiber and securely transfer the data to them and hope that whoever wants your data destroyed can't legally compel all of them at the same time and that you are not putting your friends and family members in harms way. This could also be done using multiple colocation data-centers and big servers with contracts in the names of different LLC's to avoid mapping the data back to you if the concern is that someone is trying to cancel you and your data. The daily data transfer quantity could be reduced using block level replication but that's a much bigger topic. Each server could do local volume or file level snapshots and deduplication.
Everything else that exists conflicts with at least one of your requirements in my opinion.
