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Components of SAN

Using the SAN components

The foundation that a SAN is built on is the interconnection of storage devices and servers. This section further discusses storage, interconnection components, and servers, and how different types of servers and storage are used in a typical SAN environment.


This section briefly describes the main types of storage devices that can be found in the market.

Disk systems

By being contained within a single “box”, a disk system usually has a central control unit that manages all the I/O, simplifying the integration of the system with other devices, such as other disk systems or servers.

We introduced you to what a Storage System consists of in the previous chapter. Depending on the specific functionality offered by a particular storage system, it is possible to make it behave as a small/mid-size or enterprise solution; the decision as to which type of disk system is more suitable for a SAN implementation strongly depends on the performance capacity and availability requirements for the particular SAN.

Tape systems

Tape systems, in much the same way as disk systems do, are devices that comprise all the necessary apparatus to manage the use of tapes for storage purposes. In this case, however, the serial nature of a tape makes it impossible for them to be treated in parallel, as RAID devices are leading to a some what simpler architecture to manage and use.There are basically three types of systems: drives, autoloaders and libraries, that are described as follows.

Tape drives

As with disk drives, tape drives are the means by which tapes can be connected to other devices; they provide the physical and logical structure for reading from, and writing to tapes.The foundation that a SAN is built on is the interconnection of storage devices and servers. This section further discusses storage, interconnection components, and servers, and how different types of servers and storage are used in a typical SAN environment.

Tape autoloaders

Tape autoloaders are autonomous tape drives capable of managing tapes and performing automatic back-up operations. They are usually connected to high-throughput devices that require constant data back-up.

Tape libraries

Tape libraries are devices capable of managing multiple tapes simultaneously and, as such, can be viewed as a set of independent tape drives or autoloaders. They are usually deployed in systems that require massive storage capacity, or that need some kind of data separation that would result in multiple single-tape systems. As a tape is not a random-access media, tape libraries cannot provide parallel access to multiple tapes as a way to improve performance, but they can provide redundancy as a way to improve data availability and fault-tolerance.

Once more, the circumstances under which each of these systems, or even a disk system, should be used, strongly depend on the specific requirements that a particular SAN implementation has. However, we can say that disk systems are usually used for online storage due to their superior performance, whereas tape systems are ideal for offline, high-throughput storage, due to the lower cost of storage per byte.In the next section we describe the prevalent connectivity interfaces, protocols and services for building a SAN.

SAN connectivity

SAN connectivity comprises of hardware and software components that make possible the interconnection of storage devices and servers. Now we are going to introduce you to the Fibre Channel model for Storage Area Networks.

Standards and models for storage connectivity

As we mentioned previously, networking is governed by adherence to standards and models, and data transfer is also governed by standards. By far the most common is SCSI.

SCSI is an acronym for Small Computer Systems Interface. It is an ANSI standard that has become one of the leading I/O buses in the computer industry. An industry effort was started to create a stricter standard allowing devices from different vendors to work together. This effort was recognized in the ANSI SCSI-1 standard. The SCSI-1 standard (circa 1985) is rapidly becoming obsolete. The current standard is SCSI-2, with SCSI-3 on the drawing boards.

Fibre Channel (FC) is a serial interface (primarily implemented with fiber-optic cable, and is the primary architecture for the vast majority of SANs. To support this there are many vendors in the marketplace producing Fibre Channel adapters, and other FC devices. Fibre Channel brought these advantages by introducing a new protocol stack and by keeping the SCSI-3 CCS on top of it.

Options for storage connectivity

In this section, we have divided these components into three sections accordingto the abstraction level to which they belong: lower level layers, middle level layers, and higher level layers. Figure 2-5 gives you an idea of each networking stack.

Lower level layers


As you can see there are only three stacks that can directly interact with thephysical wire, those are Ethernet, SCSI and Fibre Channel. Because of that,these models are considered as lower level layers, all the others arecombinations of them such as iSCSI, FCIP and FCoE also called middle levellayers.At this point we are assuming a basic knowledge of Ethernet which is typicallyused on conventional server-to-server or workstation-to-server networkconnections. They build up a common bus topology by which every attacheddevice can communicate with each other using this common-bus. Ethernetspeed is increasing as it becomes more pervasive in the data center. We willdescribe key concepts of Ethernet later in this book.

Middle level layers

This section comprises the transport protocol and session layers.




Internet SCSI (iSCSI) is a transport protocol that carries SCSI commands froman initiator to a target. It is a data storage networking protocol that transportsstandard Small Computer System Interface (SCSI) requests over the standard Transmission Control Protocol/Internet Protocol (TCP/IP) networking technology.

iSCSI enables the implementation of IP-based storage area networks (SANs),enabling customers to use the same networking technologies — for both storageand data networks. As it uses TCP/IP, iSCSI is also well suited to run over almostany physical network. By eliminating the need for a second network technology just for storage, iSCSI has the potential to lower the costs of deploying networkedstorage.


The Fibre Channel Protocol (FCP) is the interface protocol of SCSI on FibreChannel. It is a gigabit speed network technology primarily used for StorageNetworking. Fibre Channel is standardized in the T11 Technical Committee of theInterNational Committee for Information Technology Standards (INCITS), an

American National Standard Institute (ANSI) accredited standards committee. Itstarted for use primarily in the supercomputer field, but has become the standardconnection type for storage area networks in enterprise storage. Despite its name, Fibre Channel signaling can run on both twisted-pair copper wire and fiberoptic cables.



Fibre Channel over IP (FCIP) is also known as Fibre Channel tunneling or storage tunneling. It is a method to allow the transmission of Fibre Channel information to be tunnelled through the IP network. Because most organizations already have an existing IP infrastructure, the attraction of being able to link

geographically dispersed SANs, at a relatively low cost, is enormous.

FCIP encapsulates Fibre Channel block data and subsequently transports it over a TCP socket. TCP/IP services are utilized to establish connectivity between remote SANs. Any congestion control and management, as well as data error and data loss recovery, is handled by TCP/IP services, and does not affect FC fabric services.

The major point with FCIP is that is does not replace FC with IP, it simply allowsdeployments of FC fabrics using IP tunnelling. The assumption that this might lead to is that the “industry” has decided that FC-based SANs are more than appropriate, and that the only need for the IP connection is to facilitate any distance requirement that is beyond the current scope of an FCP SAN.


FICON architecture is an enhancement of, rather than a replacement for, the now relatively old ESCON® architecture. As a SAN is Fibre Channel based, FICON is a prerequisite for z/OS systems to fully participate in a heterogeneous SAN,where the SAN switch devices allow the mixture of open systems and mainframe traffic.

FICON is a protocol that uses Fibre Channel as its physical medium. FICON channels are capable of data rates up to 200 MBps full duplex, they extend the channel distance (up to 100 km), increase the number of control unit images per link, increase the number of device addresses per control unit link, and retain the topology and switch management characteristics of ESCON.



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