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, ...
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