IBM 3330

3830 disk control unit could attach to multiple channels

there is also head-of-string, 3333 ... that supported string-switch ... which allowed the head-of-string to connect to two different 3830 disk controllers.

there are channel programs in main memory of the processor that are pbutted by the channel to the control unit ... with the control unit executing the specified channel programs (one command at a time, in sequence). the channel architecture originally specified no-prefetching of commands (current i-o transfer command might actually modify the next channel command) which resulted in some end-to-end latency issues in various configurations.

some discussion here ... Evolution of the DASD storage control

from above ...

The 3830 in a standard configuration provided for one channel attachment and a single connection to one or two strings of DASD. (See Figure 3A.) Connectivity of a 3830 DASD subsystem could be improved by adding up to four channel switches per storage control. By means of the string-switch feature of the 3333 DASD unit, a second 3830 could be attached to a 3333. This allowed for an alternate control unit path to the DASD. An added advantage of such a configuration was an increase in the channel access capability of the DASD from four to eight channels, as shown in Figure 3B.

.. snip ..

the original nas-san might be considered the ibm mainframe disk configuration at ncar ... where they used HYPERChannel and A515 remote device adapter boxes (mainframe channel emulators) to provide distributed environment. some past posts mentioning a515s ...

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