What is the underlying file system in ASM if exists? I have gone through many sites but neither of them answers my question directly rather each site I visisted raises a different point like 'ASM uses ACFS file system' or makes the topic complicated.
Oracle ASM is both a volume manager and filesystem from Oracle database files. It does not have an 'underlying filesystem', it is the filesystem.
Oracle ASM is a volume manager and a file system for Oracle database files that supports single-instance Oracle Database and Oracle Real Application Clusters (Oracle RAC) configurations.
ACFS is something else: it's a filesystem based on ASM, but for "regular" files. It is meant as a general purpose filesystem, that happens to use ASM as a backend.
Oracle Automatic Storage Management Cluster File System (Oracle ACFS) is a multi-platform, scalable file system, and storage management technology that extends Oracle Automatic Storage Management (Oracle ASM) functionality to support customer files maintained outside of Oracle Database. Oracle ACFS supports many database and application files, including executables, database trace files, database alert logs, application reports, BFILEs, and configuration files. Other supported files are video, audio, text, images, engineering drawings, and other general-purpose application file data.