Free Tutors on Control of the media changer SCSI and Fibre Channel interface

Free Tutors on Control of the media changer SCSI and Fibre Channel interface

Applications must be able to control the media changers in automatic libraries. To this end, these libraries are equipped with suitable interfaces, which the applications use to send commands and receive return messages. In the Open Systems environment the direct connection via SCSI or Fibre Channel (in-band interface) is the most widespread. On  the other hand, proprietary out-band interfaces tend to be used more in the mainframe

environment.

SCSI and Fibre Channel interface

Two procedures have established themselves for the control of the media changer via the SCSI or Fibre Channel FCP interface. In one case the media changer is equipped with its own controller and can be addressed as a separate device over its own SCSI target ID. In the other case the media changer shares the controller with the tape drives (Figure 9.1). Then it is either visible as an independent device with separate LUN (inde- pendent media changer) or is controlled via the drive LUN using special commands (attached media changer). As is often the case in the IT world, there are two contrasting philosophies here, that reveal their specific advantages and disadvantages depending upon the application case.If the media changer shares the same controller with the tape drive, then the bandwidth available to the drive is reduced. However, as only a relatively small number of commands are transferred and carried out for media changers, the reduction of the bandwidth available for the drive is low. This is particularly true in the Open Systems environment, where tapes are predominantly used in streaming mode. If, on the other hand, access is mainly file-based and if the files are located on several tapes, the ratio of media changer to drive commands increases correspondingly. In this case it can be worthwhile conducting the communication with the media changer over an addi- tional controller. However, both this additional controller and the additional SAN com- ponents make such a solution more expensive and involve additional management costs. The addressing of the media changer over a second LUN of the drive controller has a further major advantage in addition to the low costs. Normally, several drives are fitted in a large library. Additional access paths make it possible to also control the media changer over the second LUN of a different drive controller. If a drive should fail, the media changer remains accessible. Furthermore, drives are often provided with a redundant power supply or the controllers possess an additional port, which can automatically be used if the first path fails.

can be reached as an additional device with a different LUN over the SCSI port of the drive. Attached media changers form a unit with the drive. In order to move the media changer, special SCSI commands such as

 READ ELEMENT STATUS ATTACHED and MOVE MEDIUM ATTACHED

must be used Proprietary interfaces In addition to SCSI interfaces, further interfaces have established themselves, particularly in the mainframe environment. These interfaces offer a higher level of abstraction than SCSI and often also a rudimentary management of the media. Typically, such interfaces are out-of-band, i.e. not accessible over the data path (SCSI or Fibre Channel connection), but instead over TCP/IP or RS-232 (Figure 9.2).

The commands that are exchanged over such interfaces are generally executed by a control unit that is fitted in the library. This control unit can usually accept and execute commands from several applications at the same time, without this leading to conflicts. Likewise, additional services such as the management of scratch pools can be made available to all applications.