SAN topologies can be divided by how they manage metadata -- requests for data and information about where data is stored.
The symmetrical, or storage management, approach uses a computing element in the SAN itself to handle metadata. This can be built into a SAN switch or director or a separate part of the SAN. Symmetrical pooling is easy to install, transparent to servers and operating systems and can significantly multiply the amount of storage that can be administered. However, placing the SAN manager in the computing stream also adds latency and can add complexity and cause concerns in load balancing.
The asymmetrical, or metadata, approach uses an element topologically separate from the data flow over the SAN to act as central controller. This metadata server uses a separate path between the SAN and the servers to provide mapping and other functions. Sometimes it may use a different protocol, such as Ethernet on a Fibre Channel SAN. Metadata servers can use NFS or CFS requests, which the server translates into direct block I/O operations. The other alternative uses HBA (Host Bus Adapter) or driver software to store the metadata information for the server.
The metadata server approach does all the storage virtualization in one place (the metadata server), simplifies things for the storage administrator and is transparent to the applications and operation systems. However, there is an administration effort needed for each application server and only the HBA or driver solutions can handle raw I/O.
A white paper titled "An Overview of Symmetrical and Asymmetrical Pooling" is available from the Fibre Channel Industry Association's (SNIA) web site. In it the Evaluator Group Inc. discusses the differences.
Rick Cook has been writing about mass storage since the days when the term meant an 80K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last twenty years he has been a freelance writer specializing in storage and other computer issues.