We evaluated an in-memory database a while back (Times Ten), and data was persisted to disk, just not as frequently. The database read all the data from disk into memory at startup, and wrote to disk as it changed.

An in-memory database usually means that, post-startup, the database doesn't go to disk to read.

I have a former co-worker who works for MySQL. She said that several former MySQL engineers work for Facebook, and MySQL has a team devoted just to supporting the social media company. The type of tickets Facebook tends to log with MySQL are,

"I was debugging mysql and this mutex in the whatsit is staying locked even though the whatsit should not be in use".

At Facebook, they have an understanding of MySQL that goes all the way down to the metal.

As a former MySQL and Oracle DBA, I say that you shouldn't knock it till you try it.