[chousuke]

AWS and GCP most likely use their own proprietary stuff. Various storage systems (such as Netapp) are able to provide snapshots at the storage system level, and if you're interested in something open source, a Ceph cluster can also provide you snapshottable block devices; whether it's a good idea for a database is another question.

Filesystem snapshots are a legitimate way of backing up databases, but it's not quite as simple as just taking a snapshot. For PostgreSQL for example you will still need to call pg_start_backup() and ensure your WAL archives are properly stored in your object storage system for point-in-time recovery. Without the database-specific precautions, your snapshots will still be crash-consistent and most likely usable in some manner, but not quite proper backups.

Using BTRFS or ZFS as the database filesystem has its own footguns. For example, the default record size of ZFS datasets doesn't match the block size of most databases, so if you forget to take that into account, you'll very likely see rather terrible performance.