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A Serial ATA 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.

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 / impossible).

Port multiplication

A Serial ATA port multiplier is a unidirectional splitting device. While it allows one equipped port to connect to 15 disks, the bandwidth available is limited to the 3 Gbit/s of the link to the controller.^[1] 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. 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 (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 not performance.

FIS(Frame Information Structure)–based 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 3 Gbit/s for a single SATA II-300 port. This means that only around 3 drives can be connected before the data from the drives saturates the controller port during sequential reads or writes (this assumes a single drive is capable of transferring 100 Megabytes/s, and that all drives are read/written simultaneously). For random access to spinning disks, many more drives would be necessary to saturate a single link.

References

1. ^ Port Multipliers

• www.sata-io.org
• list of SATA controllers including port multiplier capabilities