But how does the reset happen fast, the problem isn't with preventing permanent writes or w/e, it's with actually resetting for the next test. Also using overlayfs will immediately be slower at runtime than tmpfs which we're already doing.
If your db is small enough to fit in tmpfs, than sure, that is hard to beat. But then xfs and zfs are overkill too.
EDIT: I see you mentioning that starting the db is slow due to wiping and filling at runtime. But the idea of a snapshot is that you don't have to do that, unless I misunderstand you.
There's actually a potential solution here, but I haven't personally tested it: transportable tablespaces in either MySQL [1] or MariaDB [2].
The basic idea is it allows you to take pre-existing table data files from the filesystem and use them directly for a table's data. So with a bit of custom automation, you could have a setup where you have pre-exported fixture table data files, which you then make a copy of at the filesystem level, and then import as tablespaces before running each test. So a key step is making that fs copy fast, either by having it be in-memory (tmpfs) or by using a copy-on-write filesystem.
If you have a lot of tables then this might not be much faster than the 0.5-2s performance cited above though. iirc there have been some edge cases and bugs relating to the transportable tablespace feature over the years as well, but I'm not really up to speed on the status of that in recent MySQL or MariaDB.
[1] https://dev.mysql.com/doc/refman/8.0/en/innodb-table-import....
[2] https://mariadb.com/docs/server/server-usage/storage-engines...