There are three major factors that favor SAS, says Doug Coatney of the Core Systems’ Storage Engineering team at NetApp.
- Point-to-point isolation
- Increased bandwidth
- Greatly improved connectivity
Point-to-point. SAS uses a point-to point approach with nonblocking switches (expanders) similar to an Ethernet switch. These expanders provide complete isolation for each attached device, thereby avoiding many of the problems that can affect resiliency in loop-based topologies like Fibre Channel-Arbitrated Loop (FC-AL, the protocol used to connect to FC disks). SAS expanders provide path switching, arbitration and central management, and they also serve as traffic cops to make sure that all devices share bandwidth fairly.
Bandwidth. Current FC disk bandwidth is 4Gb/s per port. We have quad port PCIe adapters, so the aggregate FC-AL bandwidth per PCIe slot is 16Gb/s.
The NetApp® DS4243 product supports 3Gb/s SAS links. Looking at the same PCIe slot, a SAS quad adapter has four ports with 12Gb/s aggregate bandwidth each, for a total of 48Gb/s.
A storage-controller-to-disk-subsystem connection is made through a SAS “wide port.” The standard wide port is a set of four SAS lanes working together to provide maximum throughput. With SAS bandwidth of 3Gb/s, grouping four ports together yields a maximum bandwidth of 12Gb/s. This provides a wide pipe for storage systems to move data to and from disk subsystems. These lanes provide greater bandwidth from storage controllers to disk enclosures, plus bandwidth balancing, path redundancy, and improved error recovery, making it possible to safely scale a SAS connection to large numbers of disk drives.
Connectivity. With FC-AL, the theoretical maximum number of drives supported on a loop is 126.
For SAS, the number of drives that can be connected to a single SAS port is limited primarily by performance considerations. In a SAS domain, the use of expanders yields up to 16,384 uniquely addressed devices. A “fan-out” expander can connect to 128 edge expanders, each capable of connecting to 128 disk drives: 128 x 128 = 16,384. (These are theoretical maximums. See the following section for numbers of drives supported with the DS4243.)
Doug says also that the main difference between a SAS disk and an equivalent FC disk is in the interface electronics. SAS disk drives do not introduce new technology risks because they are mechanically equivalent to existing FC disk drives, use the same SCSI command set, and have the same performance and reliability characteristics. SAS disk drives simply use a different serial communication protocol.