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what is mysql database and sys database is about

DBA100DBA100 Current User Role Patron
I read this:
https://dev.mysql.com/doc/refman/8.0/en/sys-schema.html
it seems we can't restore mysql sys DB from percona xtradB cluster 5.7 to 8.0.19, right ?
how about "mysql" Db, it seems installed by a fresh installation?

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  • matthewbmatthewb Senior [email protected] Percona Staff Role
    Yes you can. You can absolutely restore a 5.7 backup, then install 8.0.19 and start MySQL. The databases will automatically get upgraded to 8.0.19. Downgrade back to 5.7 would be impossible.
  • DBA100DBA100 Current User Role Patron
    "You can absolutely restore a 5.7 backup, then install 8.0.19 and start MySQL."

    idea is, if I do not backup the mysql and sys dB from PXC 5.7 to 8.0.19, will it be ok ? what is inside by the way ? mysql should store username and password and grants, right? how about sys? 
  • matthewbmatthewb Senior [email protected] Percona Staff Role
    You should backup `mysql` database from 5.7 so that all of your users are recreated as well. You can ignore 'sys'; it has NOTHING to do with running MySQL. You can 100% delete `sys` and everything will work normally. The 'sys' schema is just a collection of VIEWs for use with performance_schema.
  • DBA100DBA100 Current User Role Patron
    "You should backup `mysql` database from 5.7 so that all of your users are recreated as well"
    so if I just backup mysql, should I say I don't even need this:
    pt-show-grants -uroot -pmypassword >mysqlusegrants.sql 
    to backup username and passwrod and grants?
  • matthewbmatthewb Senior [email protected] Percona Staff Role
    Please do not link to other posts asking for more free assistance.
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