The oldest Linux filesystem still viable for use now, ext2, does not have any journaling available. Therefore, any system that uses it is vulnerable to long recovery times after a crash, which makes it unsuitable for many purposes. You should not put a database volume on ext2. While that might work theoretically, there are many known situations, such as any user error made during the quite complicated fsck process, which can break the write ordering guarantees expected by the database.
Rather than presuming that you need to start with ext2, a sensible approach is to start with standard ext3, switch to write back ext3 if the WAL disk is not keeping up with its load, and only if that, too, continues to lag behind consider dropping to ...
