Some of the most important factors in obtaining High Availability are redundancy, fault tolerance and resiliency. While almost all high-end disk subsystems provide these capabilities, the same can not be said for all mid-tier disk arrays.
A typical mid-tier array consists of a couple of Storage Controllers or Storage Processors which are connected to one or more Disk Enclosures thru Fibre Channel Arbitrated Loop (FC-AL). Inside each Disk Enclosure is a backplane which the disks are connecting to. In addition, each Disk Enclosure is connected to Dual independent FC-AL loops and contains a Port Bypass Circuit (PBC). This type of Disk Enclosure is called a JBOD (Just a Bunch of Disks).
Because innovation doesn't only allow us to solve new problems but also allows us to solve old problems, a new technology surfaced around 2001 in the form of embedded FC switches (also called Switch on a Chip) in the Disk Enclosures that address the above issues. Such type of an Enclosure is called an SBOD (Switched Bunch of Disks). The embedded switches allow for each drive to have an individual FC-AL connection and be the only element in the loop thus providing faster arbitration and lower latency. In addition, the embedded switches provide the ability to isolate drive chatter without endagering the Loop. Performance and scalability are also direct beneficiaries of this technology. An SBOD, can deliver up to 50% more performance over JBOD mostly due to the fact that the bandwidth is not shared as well as due to the crossbar architecture deployed inside the embedded switches. Scalability has also improved outside of the Disk Enclosure by being able to increase the number of drives and Disk Enclosures that are connected to the FC-AL loop(s) on the Storage Controller.
Network Appliance was one of the first vendors to recognize the benefits of such technology and have been providing our customers with increased storage resiliency across all of our product line for almost 3 years now.