There aren't really any hurdles to implementing an SQL database on top of a plain block device, are there? So I wonder why no-one has gone there. This would all

There aren't really any hurdles to implementing an SQL database on top of a plain block device, are there? So I wonder why no-one has gone there. This would allow the database server to do caching in a way that makes sense for the database and not have to hope that the filesystem cache does the right thing.