Addresses and Identifiers are some of the most important parts of the Storage Networking world. These are the parts of the system that allow use to identify what entity we are trying to connect to an communicate with. If you look at the definition of “nexus” in the SAN, it is all about the identifiers of the components.
Address vs. Identifier
I just want to be a little picky here, for just a moment about terminology. Some protocols will use the term “Address”, when it really should be “identifier”. For example, in networking it is very common to use the phrase “MAC Address” when the MAC is really an identifier. It normally identifies a particular piece of hardware that is almost always at an endpoint or port to a particular device. Addresses, if you think about it, are normally used to tell us how to get somewhere, like looking up the address of a building. But very often there are multiple points-of-entry to a building (or device), so and address only gets us close to where we really want to go. <Picky mode off>
Why Do We Care?
I will often hear the above question about these addresses and identifiers. The simple answer, in most cases, is : troubleshooting! Depending on what interface and protocol we are using, we will be looking to identify what pieces of the system are involved when a problem occurs, what the source and destination device is, and what happened to cause a problem. So if you are asked what devices and pieces of the infrastructure are involved, you need to be able to find that out.
Sample Addresses & Identifiers
Parallel SCSI – Parallel SCSI uses what are called SCSI IDs. These are simply bits on a bus, in the range of 0 (zero) through 15. This bits are used during the connection sequence to select, or reselect another device on the bus.
Serial Attached SCSI (SAS) – SAS uses SAS Addresses. This is a 64-bit address that is used in the connection sequence to start (or continue) communication with another device in the SAS domain.
Fibre Channel (FC) – FC uses Source ID (S_ID) and Destination ID (D_ID) when communicating between devices. These are in the frame header.