A Serial ATA

Serial ATA

Serial ATA is a computer bus interface for connecting host bus adapters to mass storage devices such as hard disk drives and optical drives...

 port multiplier is a device that allows one to connect multiple SATA devices to a single SATA host port in a similar manner to that of a USB hub

USB hub

A USB hub is a device that expands a single USB port into several so that there are more ports available to connect devices to a host system.USB hubs are often built into equipment such as computers, keyboards, monitors, or printers...

. Many common controllers do not support this feature, as it is not a requirement for a SATA controller.

Benefits

Port multipliers have the following potential benefits:

• Reduced cable count (e.g., when using backplanes with integrated port multipliers, or external multi-drive enclosures etc.).
• Additional drives can be supported without additional SATA controllers (in some systems additional controllers may be impractical or impossible).

Port multiplication

A Serial ATA port multiplier is a unidirectional splitting device. While it allows one equipped port to connect up to 15 disks, the bandwidth available is limited to the bandwidth of the link to the controller, currently 1.5 or 3 or 6 Gbit/s. While the controller is aware that there are multiple drives connected, the service is transparent to the disks attached. Because they believe they are communicating directly with the controller, any drive that holds to the SATA standard can be connected to a port multiplier. There are two ways port multipliers can be driven:

Command-based switching

This system is similar to a mechanical A/B switch or Ethernet hub

Ethernet hub

An Ethernet hub, active hub, network hub, repeater hub or hub is a device for connecting multiple Ethernet devices together and making them act as a single network segment. A hub works at the physical layer of the OSI model. The device is a form of multiport repeater...

. The controller can issue commands to only one disk at a time and cannot issue commands to another disk until the command queue has been completed for the current transactions. This also hampers the use of Native Command Queuing

Native Command Queuing

Native Command Queuing is a technology designed to increase performance of SATA hard disks under certain conditions by allowing the individual hard disk to internally optimize the order in which received read and write commands are executed...

(NCQ). This means that the full bandwidth of the link will most likely not be used. This kind of switching is therefore used when capacity is the major concern, and not performance.

FIS–based (frame information structure) switching

FIS–based switching is similar to a USB hub. In this method of switching the host controller can issue commands to send and receive data from any drive at any time. A balancing algorithm ensures a fair allocation of available bandwidth to each drive. FIS-based switching allows the aggregated saturation of the host link and does not interfere with NCQ.

Performance

It is possible to connect 15 devices to a single SATA host port using a port multiplier, however the bandwidth is still limited to the bandwidth of a single sata port.