BASICS OF SCSI STORAGE NETWORKS

SCSI basics

The Small Computer System Interface (SCSI) was for a long time the technology for I/O buses in Unix and PC servers, is still very important today and will presumably remain so for a good many years to come. The first version of the SCSI standard was released in 1986. Since then SCSI has been continuously developed in order to keep it abreast with technical progress.

As a medium, SCSI defines a parallel bus for the transmission of data with additional lines for the control of communication. The bus can be realized in the form of printed conductors on the circuit board or as a cable. Over time, numerous cable and plug types have been defined that are not directly compatible with one another (Table 3.1). A so-called daisy chain can connect up to 16 devices together (Figure 3.3).The SCSI protocol defines how the devices communicate with each other via the SCSI bus. It specifies how the devices reserve the SCSI bus and in which format data is transferred. The SCSI protocol has been further developed over the years. For example, a server could originally only begin a new SCSI command when the previous SCSI command had been acknowledged by the partner; however, precisely this overlapping of SCSI commands is the basis for the performance increase achieved by RAID. Today it is even possible using asynchronous I/O to initiate several write or read commands to a storage device at the same time.

The SCSI protocol introduces SCSI IDs (sometimes also called target ID or just ID) And Logical Unit Numbers (LUN) for the addressing of devices. Each device in the SCSI us must have an unambiguous ID, with the host bus adapter in the server requiring its wn ID. Depending upon the version of the SCSI standard, a maximum of 8 or 16 Ids re permitted per SCSI bus. Storage devices such as RAID disk subsystems, intelligent isk subsystems or tape libraries can include several subdevices, such as virtual hard isks, tape drives or a media changer to insert the tapes, which means that the IDs would e used up very quickly. Therefore, so-called LUNs were introduced in order to address ubdevices within larger devices (Figure 3.4). A server can be equipped with several SCSI Controllers. Therefore, the operating system must note three things for the differentiation f devices – the controller ID, SCSI ID and LUN .The priority of SCSI IDs is slightly trickier. Originally, the SCSI protocol permitted nly eight IDs, with the ID '7' having the highest priority. More recent versions of he SCSI protocol permit 16 different IDs. For reasons of compatibility the IDs '7' to 0' should retain the highest priority, so that the IDs '15' to 8' have a lower priority Figure 3.5).Devices (servers and storage devices) must reserve the SCSI bus (arbitrate) before they may send data through it. During the arbitration of the bus, the device that has the highest

 

priority SCSI ID always wins. In the event that the bus is heavily loaded, this can lead to devices with lower priorities never being allowed to send data. The SCSI arbitration procedure is therefore 'unfair'.

SCSI and storage networks

SCSI is only suitable for the realization of storage networks to a limited degree. First, a SCSI daisy chain can only connect a very few devices with each other. Although it is theoretically possible to connect several servers to a SCSI bus, this does not work very well in practice. Clusters with so-called twin-tailed SCSI cables and a stand-by server have proved their worth in increasing the availability of data and the applications based upon it (Figure 3.6). Both servers can access the shared storage devices, with only one server having active access to the data at any time. If this server fails, then the stand-by server actively accesses the storage device and continues to operate the application. Second, the maximum lengths of SCSI buses greatly limit the construction of storage networks. Large disk subsystems have over 30 connection ports for SCSI cables, so that several dozen servers can access them (Figure 3.7), and many of the advan-

tags of storage-centric IT architectures can be achieved with this layout. However

due to the dimensions of disk subsystems, tape libraries and servers and the length limits of SCSI buses, constructing the configuration shown in Figure 3.7 using real devices is a challenge. Although it is possible to extend the length of the SCSI buses with so-called link extenders, the use of a large number of link extenders is unwieldy. Despite these limitations, SCSI is of great importance even for storage-centric IT systems. Techniques such as Fibre Channel SAN and iSCSI merely replace the SCSI bus by a network; the SCSI protocol is still used for communication over this network. The advantage of continuing to use the SCSI protocol is that the transition of SCSI cables o storage networks remains hidden from applications and higher layers of the operating system. SCSI also turns up within the disk subsystems and NAS servers used in storage networks.