[1] A shared-disk file system uses a storage area network (SAN) to allow multiple computers to gain direct disk access at the block level.
[2] Distributed file systems do not share block level access to the same storage but use a network protocol.
A common performance measurement of a clustered file system is the amount of time needed to satisfy service requests.
But in a clustered file system, a remote access has additional overhead due to the distributed structure.
Concurrency control becomes an issue when more than one person or client is accessing the same file or block and want to update it.
[9] This problem is usually handled by concurrency control or locking which may either be built into the file system or provided by an add-on protocol.
IBM mainframes in the 1970s could share physical disks and file systems if each machine had its own channel connection to the drives' control units.
In the 1980s, Digital Equipment Corporation's TOPS-20 and OpenVMS clusters (VAX/ALPHA/IA64) included shared disk file systems.