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A port multiplier is a device which helps in establishing a connection for communication between a single active host and multiple drives. Usually residing on the backplane of an enclosure, it supports all standard serial advanced technology attachment (SATA) drives. Externally and internally, a port multiplier provides convenient and cost-effective storage scalability to computers and servers with serial advanced technology attachment drives.
Inside the enclosure of the port multiplier, one controller, a connection to the controller and connections for drives are present. Transparency is provided to the drives by the port multiplier, although the host recognizes that it is communicating with multiple drives. The controller’s port count helps in determining the maximum number of drives in the array. There are two methods of driving port multipliers, namely command-based switching and frame information structure-based switching. Command-based switching is used when capacity is the main criteria and not performance. The controller issues commands to one disk at a time and does not proceed to another until the current transactions are complete. It is very likely to hinder the native command queuing. Frame information structure-based switching works similarly to a USB hub. In this case, the controller issues commands to any drives and can send and receive data from them at any point of time. Native command queuing is not impacted and aggregated saturation of the host link is achieved.
One of the main advantages of using port multipliers is the simplification possible using fewer cables. Connection to drives is possible for a single host adapter occupying a single peripheral component interconnect by using port multipliers. Port multipliers help in ensuring simplified drive insertion and removal as well as tidier backplanes thanks to reduced cable count. They provide cost-effective and simple storage expansion. In the case of port multipliers, additional controllers are not required for expanding the storage capacity. Compared to universal serial bus, it provides significantly higher performance in external storage.
One of the main drawbacks of the port multiplier is that the drives must have a high external serial advanced technology attachment connection.