The term "intelligent switch" implies that there's something "dumb" about other switches. The current crop of Fibre Channel (FC) switches from Brocade Communications Systems Inc., Computer Network Technology Corp. (CNT), McData Corp., QLogic Corp. and Cisco Systems Inc. aren't dumb, but they have a single-minded focus: switching a signal from one port to another as fast as possible, usually within five microseconds. These switches deliver sophisticated fabric services, such as high availability, performance monitoring and device management.
A switch is labeled "intelligent" when it can run applications that generally run on hosts or storage devices. These applications are built on a foundation layer of virtualization, such as volume management, replication, mirroring, snapshots, logical unit number (LUN) masking, backup and restore. But just because an application runs in the fabric doesn't necessarily make it better.
A typical enterprise may have storage and storage applications from a variety of vendors, all managed by an army of specialists. But consider how much simpler and less costly it would be to manage this diverse storage if it were available in a uniform fashion across all storage devices and hosts.
Different ways to embed intelligence
There are three broad platform categories for delivering storage applications from the storage area network (SAN) fabric: intelligent switches, general-purpose appliances and purpose-built appliances (PBA). The intelligent switches (or directors) share the common characteristic that there's processing power associated with each port, in addition to normal layer-2 switching functionality. This is generally provided by an additional ASIC or network processor at each port. In a director-class product, these intelligent ports are generally delivered on a blade with eight, 16 or 32 ports. In addition to the intelligent ports, the architecture generally calls for an additional blade where the application runs. This "application blade" may be as simple as a bladed version of a standard Intel processor, memory, cache, I/O, running Linux or it may be a specialty processor designed to run specific applications efficiently.
An application works with the intelligent ports to direct I/O traffic to the appropriate storage system, host or to another switch. Another crucial activity that takes place at the port level is frame termination and regeneration, or frame cracking. This essentially means that the FC frame (a multiprotocol port could also handle iSCSI, FCIP or iFCP traffic) is cracked open to obtain relevant information about the content so it can be manipulated, reformatted if necessary and then pushed off to its destination. What manipulation occurs depends on the application and could be as simple as discarding a frame not authorized to be sent to the specified destination or automatically replicating a frame for data protection depending on defined policies. Policy information is generally held in the application blade. Terminating FC traffic, cracking open FC frames and performing virtualization table look-ups require lots of processing power, so most intelligent switches add an ASIC to each port.
The level of processing power added to each port determines how much work can be done at the port level and how much must be done in the application blade. This clearly has implications for performance and scalability. So while implementations of intelligent switches vary in this dimension, fundamentally they operate the same way. Latencies incurred by applications (wherever they are hosted) will show up as switch latencies when pushing application functionality into intelligent switches. Whereas a typical FC switch adds about five microseconds of latency, an intelligent switch will add about 25 microseconds of delay.
