Port Multiplier is a device that expands the number of devices to be installed on a single SATA port to 15.
Port multiplier has several applications, like allowing a home user to install more than one hard disk drive per SATA port and allowing storage racks to use fewer cables.
With Serial ATA it is easier to connect hard disk drives located outside the computer case at a high-speed rate because of the cable that is used (which is thinner than the traditional 80-wire flat cable). But if we need to install a rack containing 16 hard disk drives to a server, there will be 16 Serial ATA cables from the rack to the server, and the server must have 16 SATA ports. We illustrate this scenario in Figure 10.
Using port multiplier it is possible to connect them using fewer cables. For example, one port multiplier connected to one SATA port allows you to connect up to 15 hard disk drives to it. And you would have only one cable connecting the rack to the server.
But there is a huge performance issue here. If a SATA-150 port were used, the 150 MB/s bandwidth would have to be split between 15 devices, creating a huge bottleneck.
To solve this issue another approach may be used. Instead of using only one port multiplier chip, you could use four of them, connecting the rack to the server using four cables (instead of 16). The maximum transfer rate between the server and the rack would be of 600 MB/s (4x 150 MB/s) if SATA-150 ports were used or of 1,200 MB/s (4x 300 MB/s) if SATA-300 were used. Inside the rack, you could install up to 60 hard disk drives (15 x 4), but for optimal performance you should install four hard disk drives to each port multiplier chip, matching your 16 drives. We illustrate this scenario in Figure 11. “PM” there is the port multiplier chip.