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Serial ATA Port Multiplier Technology:
Features and Benefits of
Expanded SATA PM Connectivity
What are port multipliers?
Port multipliers are silicon-based devices that allow a single Serial ATA port to communicate with multiple drives.
Where are they used?
Port multipliers typically reside on an enclosure’s backplane and are transparent to the drives. Port multipliers support all standard SATA drives.
What value do they bring to the customer?
Port multipliers allow cost-effective and expanded drive scalability to storage systems. Simplified cabling allows the host to be connected to up to fifteen SATA devices by a single cable.
Key message
A single host adapter occupying a single PCI slot is able to connect up to five times as many drives with no performance degradation on a 3Gb/s line.
Until now Serial ATA (SATA) connectivity between drives and controllers has been an effective point-to-point relationship; a single drive connected to a single controller port via a single cable. The maximum number of drives in an array was predicated on the controller’s port count. The SATA Port Multiplier (SATA PM) specification permits a change to that point-to-point relationship with the introduction of port multiplication technology. Port multipliers allow easy, cost-effective storage expansion and push SATA’s performance reach from sequential applications only to random applications as well.
A port multiplier is a silicon-based unidirectional fan-out device, typically residing on an enclosure’s backplane. It enables one host SATA PM enabled port to be connected to multiple SATA drives, similar to USB connectivity, but with the performance benefits of an aggregated switch. The port multiplier is transparent to the drives. The host knows that it is communicating to multiple drives, but the drives are unaware that they are being multiplexed. The SATA drives function as if they were directly attached to the host adapter. Port multipliers support any standard SATA drive.
While it is possible to connect up to 15 drives to each SATA PM port via a port multiplier, drive connectivity is practically limited to the maximum available bandwidth on the 3Gb/s link. Sustained I/O rates from the drives are kept to within the 3Gb/s host port connection limit for maximum efficiency and performance.
Benefits of Port Multipliers
Port multipliers allow cost effective and expanded drive scalability to storage systems both inside and outside the box. Efficient add-on desktop storage with significantly higher performance than Firewire™ and USB 2.0 using external connections is assured. The number of extra drives that can be added to a conventional SATA system without port multipliers is limited to the controller’s port count; additional drives mean additional controllers, effectively increasing the cost of the system. The user pays the cost of the extra controller(s) and necessarily forfeits additional PCI slots that may otherwise be needed for other peripheral upgrades. By using port multipliers, a single host adapter occupying a single PCI slot is able to connect four times as many drives with no performance degradation on a 3Gb/s line.
SATA PM’s simplified cabling topology where the host is connected to more drives by fewer cables is another plus for port multiplier connectivity. SATA’s point-to-point relationship in which each port is connected to a single drive via a single cable means overly complicated cabling for multi-drive systems. A reduced cable count contributes to tidier backplanes, simpler drive insertion and removal, improved airflow inside the box, and a more secure system.
Command-based or FIS-based Host Support
SATA PM requires that controllers support either command-based switching or FIS (Frame Information Structure)-based switching in order to use port multiplication. Each paradigm offers unique capabilities suited for particular environments.
Command-based switching, conceptually similar to a mechanical A/B switch, limits the host to issue commands to only one drive at a time. Commands to other drives will not be issued until the command queue is completed for the prior transaction. Since command-based switching only accesses one drive at a time, it does not take advantage of the higher speed 3Gb/s host link. Therefore, command-based switching is ideal for simple drive expansion where capacity is more important than performance.
FIS–based switching offers high performance storage connections to multiple drives simultaneously. The host issues and completes commands to drives at any time. The port multiplier will direct data to any drive ready for I/O. An arbitration algorithm ensures a balanced data flow. Unlike Command-based switching, FIS-based switching allows aggregation of reads to fully use the higher bandwidth of the 3Gb/s host link and takes full advantage of the performance benefits of Native Command Queuing (NCQ) on the port multiplier, resulting in aggregated throughput of up to 300MB/second.
Conclusion
Port multipliers are simple mechanisms that allow a single active host connection to communicate with multiple drives. They allow easy, cost-effective storage scalability both inside and outside the PC or server with standard SATA drives. Additional controllers are no longer necessarily needed to expand storage. In-box cabling is greatly simplified with fewer cables attached to more drives. Port multipliers allow significantly higher performance in external storage than either USB 2.0 or Firewire™.