File systems either access storage devices directly or access virtual disks
presented by a volume manager. Database management systems typically
store data either in container files, in managed volumes that they perceive as
disks, or directly on disks or disk partitions.
Volume managers were originally targeted to provide RAID functionality in
situations where it was not possible or desirable to use hardware RAID
controllers. The volume manager, as a server software component, forms a
layer between file systems and HBA drivers. The main purpose of volume
managers is to virtualize whatever the server perceives as physical disk
drives, presenting it in a form more convenient for file system use.
Note that a volume manager does not perform the access restrictions of a file
system. Any application or data manager that can secure the right to access a
volume from its operating system is permitted to read or write any block in the
volume’s user data area.