As it pertains to storage, it is the ability to access a particular LUN from either two different paths on a single host simultaneously (load balanced), or by two separate hosts within a cluster environment (distributed lock management).
The problem to overcome is when two hosts try to "change" the data at the same time. This is where a lock manager is needed to allow cooperation between the hosts for data access. Within a storage array, using dual ported fibre drives, the firmware can load balance a drive over both ports, and "queue" I/O across the "least full" path. This is a form of "concurrent access" also.
Things get real interesting when you use a "universal" file system. By universal, I mean a file system that is SAN based and is understood by all operating systems. This is very similar to file based access protocols like NFS and CIFS, but is "block" based for high performance applications. There are many vendors working on this type of solution, but it will need the involvement of ALL the OS vendors. This will mean cooperation between SUN (Solaris), Microsoft (NT, Win2K), IBM(AIX), HP(HP-UX), Compaq (VMS, True64 Unix), and all the Linux variants. Something this broad will need to be driven by a standards body like SNIA, FCIA, IETF or the IEEE.
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This was first published in November 2001