Port multiplier


Port multiplier

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 a few major benefits:
* Reduced cable count / Tidier cabling.
* Lower cost for large drive arrays due to reduced controller count.
* No need to use slots on the computer's bus to increase the number of connectible drives.

Port multiplication

A Serial ATA port multiplier is a unidirectional splitting device it allows one equipped port to connect to 15 disks, however the bandwidth available is limited to the 3 Gbit/s of the link to the controller. While the controller is aware that there are multiple drives connected, the service is transparent to the disks attached. They believe they are communicating directly with the controller, this way 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. This means that realistically only around 3 drives can be connected before the data from the drives saturates the controller port.

References

* [http://www.sata-io.org/portmultiplier.asp www.sata-io.org]


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