Yes, ZFS snapshots are atomic, so as long as the database use sane semantics to do disk I/O, you're fine (AFAIK, PostgreSQL and MySQL/InnoDB are ok, MySQL/MyISA

Yes, ZFS snapshots are atomic, so as long as the database use sane semantics to do disk I/O, you're fine (AFAIK, PostgreSQL and MySQL/InnoDB are ok, MySQL/MyISAM is not though). A snapshot is effectively identical to a killed database server (due to OOM, or power outage), and database servers should be crash-resistant. With PostgreSQL, checkpointing the database before taking the snapshot may make the crash recovery quicker when loading the snapshotted database.