Making Sense of ANs and ASes




Each of these acronyms contain the letter "A". In the case of DAS, & NAS the "A" represents "Attached". The "S" represents "Storage".

Figure 1: Basic Storage Networking Technologies

With DAS, the Application, the File System and the Storage are often resident with or connected to the same system. This might include:

  • A laptop computer with a Serial ATA connection to an internal hard drive.
  • A blade printed circuit connecting to a SCSI drive to the CPU on the same blade.
  • An Ethernet or USB cable to a connected storage device from a system.

The term DAS suggests function and the main theme is the attachment to a single server with the connection.

With NAS we are distributing the File System and Storage component over a network infrastructure enabling file sharing amongst multiple clients where the NAS function allows heterogeneous shared access to a common file system. The term NAS itself is a misnomer. NAS would have been more accurately called "NAFS" for Network Attached File Services. The NAS itself may have DAS or SAN connectivity to storage but it is the remote File Services the client workstations and servers are interested in.

With SAN we are distributing storage only over a network infrastructure enabling access to storage over a network infrastructure, often switches and links enabling access to a common set of storage space. This space can be configured into Logical Units of contiguous free space. The information in this LU may be shared with the right software.

One confusing letter here is the "A" in SAN. In this case the "A" symbolizes "Area" which suggested an inappropriate relationship to geography and not functionality. Geography comes in with our next set of acronyms.

Figure 2: SmAN, LAN, MAN, WAN

In Figure 2 we see the concept of LAN, MAN, WAN and SmAN. Here the "A" represents "Area" and the "N" represents Network. A Local Area Network (LAN) enables multiple systems and storage to access each other over a smaller geography. Yes, I am saying that a SAN can exist as a LAN. In fact a SAN can exist as a MAN, WAN or SmAN. A SmAN would be a Small Area Network making the geography even smaller. For example the Internal SAN that provides the network connectivity from the controllers in a RAID Array to the back end disk drives using Fibre Channel Arbitrated Loops or SAS Expanders would be a SmAN.

The LAN concept can be extended to larger geographies with the concept of Virtual LANs (VLAN) however VLANs are usually using switches and connectivity over MANs and WANs. These traditional geographic boundaries blur when the same performance characteristics can be exhibited regardless of geography. Metropolitan Area Networks and Wide Area Networks relate to larger and larger geography however in most cases we are providing an appearance of media with each. An appearance of media provides a virtual path that connects one system to another such that geography is taken out of consideration and the data structure that is built to move information from one system to another (frame) is addressed from one port to the other. Certainly there are all kinds of WANs from a performance point of view. The old adage "You get what you pay for" comes to mind.

Somewhere along the way the concept of "AN" and geography became confused with function. Perhaps this was to reduce the number of acronyms that come into play but it has led to the confusion that compares SAN to LANs. When we think about early File sharing through Network File Services (NFS), the assumption was that this was a LAN protocol. Initially it was and used User Datagram Protocol (UDP) transport. However NFS has supported Transmission Control Protocol (TCP) for a long time allowing File Sharing between systems that are cross country and multiple subnets away; even over the Internet. In other words, File Services is the function, over a geography including LANs & WANs.

The connectivity for DAS, NAS and SAN whether a single cable or a network cloud could be over LAN, MAN, WAN or even SmAN. Even a single cable in a DAS environment can be viewed as a point-to-point network topology. All network topologies also including Bus, Loop, and Switched can be used in all geographies today.

In conclusion, it is important to distinguish function from geography. In this way we can truly understand the differences between or "ANs" and our "ASes" or perhaps our "ASes" and our elbows!

Howard Goldstein
President Howard Goldstein Associates, Inc.

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