Database is 20 Go. 3 m + SKUs. What Data Migration tool should I use ? Also, can I migrate Data from Live server or should I use Data from a non Live backup server ?
Answer to question 1 - You can use official migration tool from Magento Link
Second, You should always run dry migration on copy of live database, so it will give all possible error and you can find corresponding solution of those errors.
For more detail you can refer migration tool document
Hope above will help!
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As far as I know, you can migrate your data from either your live server or your backup server. It’s just that you should prepare backup data just in case your migration has errors and you need to track them down.