About Port Multiplication

  Port Multiplication is a technology to connect multiple hard drives to a single port on a PCI host card. This discussion is specific to SATA buses and drives and current technologies using SATA Port Multipliers connected to SATA PCI, PCI-X and PCI-Express host cards for Macs and PCs.

  Port Multiplication is new to the realm of SATAII Specification Host Bus Adapters. ONLY a motherboard bus, Host Bus Adapter or RAID Card that specifically supports Port Multiplication will successfully connect through a Port Multiplier card. Mounted in your hard drive enclosure a single port multiplier card supports up to 5 connected hard drives. Every port on the PCI host card or RAID controller can be connected to a port multiplier with 4 or 5 drives attached making for HUGE storage capacities with superior performance.

  Port Multiplier capable hosts are available for PCI Express slots like the excellent Sonnet Tempo E4P and CalDigit FASTA-4e host cards. For PCI and PCI-X equipped computers there is the Sonnet Tempo X4P, CalDigit FASTA-4x and the Lycom cards. Check out our SATA Host Card Guide for the appropriate card for your specific computer. Some cards work better with some computers than others.

  Installation is very simple, especially in external enclosures equipped with a port multiplier. A PCI, PCI-X or PCIe port multiplier capable host card is installed in the computer, a single data cable connects from that card to the back of the enclosure. (eSATA connectors only) In the enclosure each drive has a data cable attaching it to the port multiplier, up to 5 per port multiplier. Drives attached via a port multiplier can be mounted, dismounted and formatted the same as if they were individually connected. A portion of the drives or all of them can be formatted into RAID arrays or mixed as JBOD disks or even included in RAID arrays containing drives on other port multipliers making for monster arrays at mind blowing speeds.

  Cost efficiency is one of the overriding benefits to using port multiplication. By using many smaller and less expensive hard drives to make up large capacity storage, costs per GB go way down as opposed to the limited one drive per port method. It is far cheaper to build a 2 TB array using 8 x 250 GB drives (at 1/2 the per GB price), compared to the same capacity using 4 x 500 GB drives. With 20 drives per host card possible, smaller drives become practical and very cost effective.

  We ran performance tests with identical drives attached both via traditional individually connected drives as well as drives attached via the port multiplier. A single drive performs nearly identically. Obviously the 300 MB/sec bandwidth of a single data cable through which the port multiplier connects will hold back simultaneous transfers when you add enough drives to it. But in most cases we found the performance very good to great even in a 5 drive striped RAID array.

  Striped RAID (Software RAID0, one of the simplest and fastest types of RAID) performance is a little harder to compare. The first thing to consider is that as a hard drive fills up it looses significant performance. Drives connected individually are given access to full bandwidth all the time while drives connected via a port multiplier will only be as fast as the maximum the port multiplier can pass through a single data cable to the host card. We found that the maximum a port multiplier can move with the current state of the art of its chipset is closer to 225 - 230 MB/sec. While this will impact a 5 drive array that is empty and at its fastest, it has much less if any effect on the array's speed as it reaches 50 percent of capacity. In fact the effect is to start out at 225 MB/sec and stay there until the speed of the array drops below that the port multiplier is capable of. A striped RAID with its drives directly connected, each on its own cable/channel, will start out very fast and slow rapidly until it somewhere around 50 percent of capacity matches the speed of the port multiplier array for the rest of its total capacity.

  For Video Storage there is no better solution. Even the most demanding format, 10bit 1080i uncompressed HD, is easily handled by 8 or 10 drive port multiplier arrays. Starting out at about 450 MB/sec these arrays will maintain a constant speed up to about 65% capacity for the 8 drive array and about 80% capacity for the 10 drive array. At that point the array will begin to slow below the maximum speed as the drives themselves slow down.