Basics Hardware components for Fibre Channel SAN

Basics Hardware components for Fibre Channel SAN

Within the scope of this book we can only introduce the most important product groups. It is not worth trying to give an overview of specific products or a detailed description of individual products due to the short product cycles. This section mentions once again some product groups that have been discussed previously and introduces some product groups that have not yet been discussed. It is self-evident that servers and storage devices are connected to a Fibre Channel network. In the server this can be achieved by fiting the host bus adapter cards (HBAs) of different manufacturers, with each manufacturer offering different HBAs with differing performance features. In storage devices the same HBAs are normally used. However, the manufacturers of storage devices restrict the selection of HBAs. Of course, cables and connectors are required for cabling. In Section 3.3.2 we discussed different copper and fiber-optic cables and their properties. Various connector types are currently on offer for all cable types. It may sound banal, but in practice the installation of a Fibre Channel SAN is sometimes delayed because the connectors on the cable do not fit the connectors on the end devices, hubs and switches and a suitable adapter is not

to hand. A further, initially improbably, but important device is the so-called Fibre Channel-to- SCSI bridge. As the name suggests, a Fibre Channel-to-SCSI bridge creates a connection between Fibre Channel and SCSI (Figure 3.30). These bridges have two important fields of application. First, old storage devices often cannot be converted from SCSI to Fibre Channel. If the old devices are still functional they can continue to be used in the Fibre Channel SAN by the deployment of a Fibre Channel-to-SCSI bridge. Second, new tape libraries in particular often initially only support SCSI; the conversion to Fibre Channel is often not planned until later.With a Fibre Channel-to-SCSI bridge the newest tape libraries can be operated directly in a Fibre Channel SAN and Fibre Channel connections retrofitted as soon as they become available. Unfortunately, the manufacturers have not agreed upon consistent name for this type of device. In addition to Fibre Channel-to-SCSI bridge, terms such as SAN router or storage gateway are also common. The switch is the control centre of the fabric topology. It provides routing and aliasing, name server and zoning functions. Fibre Channel switches support both cut-through routing and the buffering of frames. In new switches a number of ports between eight and about 250 and a data transfer rate of 200 MByte/s should currently (2003) be viewed as standard. In Fibre Channel SANs that have already been installed, however, a large base of switches exists that still work at 100 MByte/s.

 

 

Resilient, enterprise-class switches are commonly referred to as 'directors', named after the switching technology used in mainframe ESCON cabling. Like Fibre Channel switches they provide routing, alias names, name server and zoning functions. Fibre Channel direc- tors are designed to avoid any single point of failure, having for instance two backplanes and two controllers. Current directors (2003) have between 64 and 256 ports. Designing a SAN often raises the question whether several complementary switches or a single director should be preferred. As described, directors are more fault-tolerant than switches, but they are more expensive per port. Therefore, designers of small entry-level SANs commonly choose two complementary Fibre Channel switches, with mutual traffic fail-over in case of a switch or a I/O path failure (Figure 3.31). Designers of larger Fibre Channel SANs often favour directors due to the number of ports currently available per device and the resulting layout simplicity. However, this argument in favour of directors becomes more and more obsolete since today switches with a greater number of ports are available as well. SANs running especially critical applications, e.g. stock market banking or flight control,

would use complementary directors with mutual traffic failover, even though these directors already avoid internal single points of failure. This is similar to wearing trousers with a belt and braces in addition: protecting against double or triple failures. In less critical cases, a single director or a dual complementary switch solution will be considered sufficient. If we disregard the number of ports and the cost, the decision for a switch or a director in an Open Systems Fibre Channel network primarily comes down to fault-tolerance of

 

an individual component. For the sake of simplicity we will use the term 'Fibre Channel switch' throughout this book in place of 'Fibre Channel switch or Fibre Channel director'. A hub simplifies the cabling of an arbitrated loop. Hubs are transparent from the point of view of the connected devices. This means that hubs send on the signals of the connected devices; in contrast to a Fibre Channel switch, however, the connected devices do not communicate with the hub. Hubs change the physical cabling from a ring to a star-shape. Hubs bridge across defective and switched-off devices, so that the physical

ring is maintained for the other devices. The arbitrated loop protocol is located above this cabling. Hubs are divided into unmanaged hubs, managed hubs and switched hubs. Unman- aged hubs are the cheap version of hubs: they can only bridge across switched-off devices. However, they can neither intervene in the event of protocol infringements by an end device nor indicate the state of the hub or the arbitrated loop to the out- side world. This means that an unmanaged hub cannot itself notify the administrator if one of its components is defective. A very cost-conscious administrator can build up a small SAN from PC systems, JBODs and unmanaged hubs. However, the upgrade path to a large Fibre Channel SAN is difficult: in larger Fibre Channel SANs it is questionable whether the economical purchase costs compensate for the higher administration costs. In contrast to unmanaged hubs, managed hubs have administration and diagnosis functions like those that are a matter of course in switches and directors.Managed hubs monitor the power supply, serviceability of fans, temperature, and the status of the individual ports. In addition, some managed hubs can, whilst remaining invisible to the connected devices, intervene in higher Fibre Channel protocol layers, for example, to deactivate the port of adevice that frequently sends invalid Fibre Channel frames. Managed hubs, like switches and directors, can inform the system administrator about events via serial interfaces, Telnet, HTTP and SNMP (see also Chapter 8). Finally, the switched hub is mid-way between a hub and a switch. In addition to the properties of a managed hub, with a switched hub several end devices can exchange data at full bandwidth. Fibre Channel switched hubs are cheaper than Fibre Channel switches, so in some cases they represent a cheap alternative to switches. However, it should benoted that only 126 devices can be connected together via hubs and that services such as aliasing and zoning are not available. Furthermore, the protocol cost for the connection or the removal of a device in a loop is somewhat higher than in a fabric (keyword 'Loop Initialisation Primitive Sequence', 'LIP'). Finally, so- alled link extenders should also be mentioned. Fibre Channel supports a maximum cable length of several ten kilometres (Section 3.3.2). A link extender can

increase the maximum cable length of Fibre Channel by transmitting Fibre Channel frames using MAN/WAN techniques such as ATM, SONET or TCP/IP (Figure 3.32).When using link extenders it should be borne in mind that long distances between end devices significantly increase the latency of a connection. Time-critical applications such as database transactions should therefore not run over a link extender. On the other hand,Fibre Channel SANs with link extenders offer new possibilities for applications such as back-up, data sharing and asynchronous data mirroring.

Fibre Channel SAN is a comparatively new technology. In many data centres in which Fibre Channel SANs are used, it is currently (2003) more likely that there will be several islands of small Fibre Channel SANs than one large Fibre Channel SAN (Figure 3.33).Over 80% of the installed Fibre Channel SANs consist only of up to four Fibre Channel switches. A server can only indirectly access data stored on a different SAN via the LAN and a second server. The reasons for the islands of small Fibre Channel SANs are that they are simpler to manage than one large Fibre Channel SAN and that it was often unnecessary to install a large one.

Originally, Fibre Channel SAN was used only as an alternative to SCSI cabling. Until now the possibility of flexibly dividing the capacity of a storage device etween several servers (storage pooling) and the improved availability of dual SANs have been the main reasons for the use of Fibre Channel SANs. Both can be realized very well with several small Fibre Channel SAN islands. However, more and more applications are now exploiting the possibilities offered by a Fibre Channel SAN. Applications such as back-up

 

(Chapter 7), remote data mirroring and data sharing over Fibre Channel SAN and storage virtualization (Chapter 5) require that all servers and storage devices are connected via a single SAN. Incidentally, the connection of Fibre Channel SANs to form a large SAN could be one field of application in which a Fibre Channel director is preferable to a Fibre Channel switch (Figure 3.34). As yet these connections are generally not critical. In the future, however, this could change (extreme situation: virtualization over several data centres). In our opinion these connection points between two storage networks tend to represent a single point of failure, so they should be designed to be particularly fault-tolerant.