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Perhaps I misunderstood:

"Using mixed storage engines is not recommended in production because it is not really transactional, ..."

http://myrocks.io/docs/getting-started/#migrating-from-innod...

I believe that is saying that MySQL transactions across two storage engines are not transactional, as that would require two-phase-commit (2PC) or similar. It might also have implications for replication.

MyRocks itself definitely has transactions. As the wiki explains, it uses snapshots (MVCC) for transaction isolation.

MySQL does have a system for two phase commit. I've seen it accidentally triggered on a system that executed queries that joined two different engines (InnoDB and TokuDB) and had the binlog turned off. It's called XA mode and it requires an fsync per commit to a transaction log file.

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