A Lesson in Multiplication

SATA has made a big impact in the storage industry, boasting high transfer speeds and great pricing. However many people are still in the dark about port multipliers and how they can save users even more money whilst expanding their storage requirements. MRi takes a stab to address this situation here.

What are port multipliers?

Quite simply a port multiplier is a little bit of kit that can allow a single port to communicate with multiple devices, commonly four or five.

A single host adapter occupying a single PCI slot is able to connect up to five times as many drives with no performance degradation on a 3Gb/s line.

Where are they used?

Port multipliers can be found on an enclosure’s backplane, in storage racks or even internally. They are transparent to the host and devices. They work irrespective of SATA specification and configuration.

What are the benefits for the user?

Port multipliers allow cost-effective drive scalability to any storage system. Simplified cabling allows the host to be connected to up to fifteen SATA devices in a tidy manageable fashion over a single host port.

Until now Serial ATA (SATA) connectivity between drives and controllers has been an effective point-to-point relationship (a single drive connected to a single controller port via a single cable), therefore, the maximum number of drives in an array was dictated by the number of ports on the controller. The SATA Port Multiplier (SATA PM) specification permits a change to that point-to-point relationship with the introduction of port multiplication technology. Port multipliers allow easy, cost-effective storage expansion that can migrate from sequential applications only to random applications as well.

In effect port multiplier is a unidirectional fan-out device. It enables one host SATA PM enabled port to be connected to multiple SATA drives, similar to USB connectivity, but with the performance benefits of an aggregated switch. The port multiplier is transparent to the drives. The host knows that it is communicating to multiple drives, but the drives are unaware that they are being multiplexed. The SATA drives function as if they were directly attached to the host adapter. Port multipliers support any standard SATA drive.

While it is possible to connect up to 15 drives to each SATA PM port via a port multiplier, drive connectivity is practically limited to the maximum available bandwidth on the SATA link permits. Sustained I/O rates from the drives are kept to within the transfer speed permitted for maximum efficiency and performance.

How the benefits are achieved

Port multipliers offer a cost effective method for drive scalability within storage systems, inside and outside the box. It permits an efficient way to add-on storage with significantly higher performance than Firewire^TM and USB 2.0. With SATA PM technology, storage scalability can be achieved without having to purchase and install additional host controllers, dramatically reducing the cost of the system and does not have to forfeit any PCI slots available for future peripheral upgrades. By using port multipliers, a single host adapter occupying a single PCI slot is able to connect four times as many drives with no performance degradation on a SATA line.

SATA PM’s simplified cabling topology where the host is connected to more drives by fewer cables is another plus for port multiplier connectivity. SATA’s point-to-point relationship in which each port is connected to a single drive via a single cable means overly complicated cabling for multi-drive systems. A reduced cable count contributes to tidier backplanes, simpler drive insertion and removal, improved airflow inside the box, and a much tidier system.

Command-based or FIS-based Host Support

SATA PM requires that controllers support either command-based switching or FIS (Frame Information Structure)-based switching in order to use port multiplication. Each paradigm offers unique capabilities suited for particular environments.

Command-based switching, conceptually similar to a mechanical A/B switch, limits the host to issue commands to only one drive at a time. Commands to other drives will not be issued until the command queue is completed for the prior transaction. Since command-based switching only accesses one drive at a time, it does not take advantage of the higher speed 3Gb/s host link. Therefore, command-based switching is ideal for simple drive expansion where capacity is more important than performance.

FIS–based switching offers high performance storage connections to multiple drives simultaneously. The host issues and completes commands to drives at any time. The port multiplier will direct data to any drive ready for I/O. An arbitration algorithm ensures a balanced data flow. Unlike Command-based switching, FIS-based switching allows aggregation of reads to fully use the higher bandwidth of the 3Gb/s host link and takes full advantage of the performance benefits of Native Command Queuing (NCQ) on the port multiplier, resulting in aggregated throughput of up to 300MB/second.

To Summarise

Port multipliers are simple mechanisms that allow a single active host connection to communicate with multiple drives. They allow easy, cost-effective storage scalability both inside and outside the PC or server with standard SATA drives. Additional controllers are no longer necessarily needed to expand storage. In-box cabling is greatly simplified with fewer cables attached to more drives. Port multipliers allow significantly higher performance in external storage than either USB 2.0 or Firewire^TM

How MRi can get you up and running

The sections below highlight which MRi high performance products offer these features to you.

Command Based Host Adapters

MRI-eSATA-4MCR -- PCI 2.2 4 port eSATA RAID 0, 1, 0+1 over Type 4x Multilane SATA connector with thumbscrews
MRI-eSATA-II-3/1P -- PCI 2.3 3x internal, 1x external port SATA II controller
MRI-eSATA-II-e1/1P -- PCIe x1 2 external port eSATA II controller
MRI-SATA-II-4R5 -- PCI 2.3 4 port SATA RAID 0, 1, 5 and JBOD controller with Low Profile Bracket

FIS Based Host Adapters

MRI-eSATA-II-e1/1R5 -- PCIe x1 2 external port eSATA II RAID 0, 1, 5 and JBOD controller
MRI-eSATA-II-e2R5 -- PCIe x1 2 external port eSATA II RAID 0, 1, 5 & JBOD controller
MRI-eSATA-II-X64-4MR5 -- PCI-X 64Bit 4 external Port SATA (II) RAID 0, 1, 5 and JBOD over Type 4x Multilane SATA connector with thumbscrews
MRI-eSATA-II-X64-4R5 -- PCI-X 64Bit 4 external Port SATA (II) RAID 0, 1, 5 and JBOD
MRI-SATA-II-e2P -- PCIe x1 2 port SATA II controller
MRI-SATA-II-e2R5 -- PCIe x1 2 port SATA II RAID 0, 1, 5 & JBOD controller
MRI-SATA-II-X64-4R5 -- PCI-X 64Bit 4 Internal Port SATA (II) RAID 0, 1, 5 and JBOD