Most file systems make use of an underlying
data storage device that offers access to an array of fixed-size physical
sectors, generally a
power of 2 in size (512
bytes or 1, 2, or 4
KiB are most common). The file system is responsible for organizing these sectors into
files and
directories, and keeping track of which sectors belong to which file and which are not being used. Most file systems
address data in fixed-sized units called "
clusters" or "
blocks" which contain a certain number of
disk sectors (usually 1-64). This is the smallest amount of disk space that can be allocated to hold a file. However, file systems need not make use of a storage device at all.