The arrangement of files into a hierarchy of folder-names anchored with "/" is a logical
arrangement of information that is used throughout all Unix variants. This may or may not correspond to the physical
arrangement of the data.
In many systems, it does correspond: the various top-level directories such as /usr, /boot, /opt, /temp
may well reside on different devices and/or partitions, and all
of the information under that particular directory reside on the same physical volume. This can be done for several reasons, such as: protection against device failure; ease of upgrading; physical location and/or type of media.
But... it ain't necessarily so. Subsystems like LVM
(Logical Volume Management) are commonly used on severs. (See for example this very complete two-part article from IBM.
Part two is here.
The LVM HOWTO from The Linux Documentation Project (TLDP) is here.
In this scenario, each "logical" volume is stored on one or several
physical volumes. Space can be added, reallocated, and removed if necessary, all without rebooting the system. Files can be distributed among several physical volumes at the same time, perhaps to gain the advantage of distributing the I/O load among different devices instead of creating a bottleneck of continuous access against just one. Some LVM management systems are quite sophisticated, with the ability to maintain backup copies of the same file on different volumes at once; to migrate frequently-used files to faster volumes; to take a "system-wide" view in cases where more than one computer-system in a cluster is accessing the same information.