Section 2.4. The Emergence of HBAs

   

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2.4 The Emergence of HBAs

Originally, storage devices such as disk or tape units were connected directly to the server. The storage units would be either within the server cage itself or housed in external units, yet directly attached to the server via the server I/O controller. The server I/O controller could be either in an add-on card or on the motherboard itself. Over the years , the term HBA , or host bus adapter (an adapter that interfaces with the I/O bus on the host), came to be used for controllers.

Figure 2.1 shows a server with an I/O controller card and several devices connected to the controller card via a SCSI bus. Direct-attached storage works well for small isolated LANs (local area networks), such as a small departmental LAN, but does not scale well at all. One obvious problem is the limitation on the number of storage units that can be used. The other problem is the fact that all I/O must be accomplished through the single SCSI bus.

Figure 2.1. Direct-Attached SCSI Storage

The next thing the storage industry developed was a storage subsystem with its own I/O controller (see Figure 2.2).

Figure 2.2. Intelligent Storage Subsystem

The storage subsystem added some complexity, as well as increased functionality, and caused some rethinking in terms of the terminology. The system in Figure 2.1 has only one I/O controller, which is connected to the server. The system in Figure 2.2 has two controllers, resulting in potential confusion as to which one is being referred to. If more storage subsystems were present, each with its own controller, things would be even more confusing. The terminology adopted was to call the controller card connected to the server a host bus adapter , or HBA . An HBA may provide connectivity to a SCSI or IDE bus or a Fibre Channel network.

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