Why you should avoid Port Multiplication

Port multiplication (PM) technology shares a single 3-gigabit SATA cable to connect multiple hard drives, typically up to five.

Unfortunately, as few as two of today’s fast hard drives can saturate the bandwidth of that single cable! Performance is limited to the ~240MB/sec range, even though five drives might be capable of 600MB/sec or more of sustained data transfer. All PM enclosures top out at around 200-230MB/sec, regardless of the number of drives. That’s because they send all data over a single eSATA cable.

This performance limitation is only relevant on the Mac Pro, since the MacBook Pro itself limits eSATA speedsd to well below the speed of a single SATA cable.

Port multiplication technology is perfectly fine if 240MB/sec is all the performance you require and/or your main goal is to create a very large volume by striping multiple hard drives.

Update! Throughout 2010, expect to see new options emerge that use eSATA 6, providing up to twice the performance of eSATA 3 with a suitable expansion card, and all but eliminating the single cable bottleneck as a limitation for most every use. See the MAXPower eSATA 6G review.

You can work around the limitations of port multiplication by using two or three port-multiplied enclosures, each with its own cable. But unless you need a huge amount of storage it’s rather silly: you might as well choose a single non-PM enclosure with 2 or 4 drive bays (see Recommended Hardware page).

For high performance, avoid external eSATA enclosures using “port multiplication” (PM) unless you understand and accept their limitations: in the quest for a single cable instead of four, you’re stuck with vastly lower performance.

There is the argument that PM enclosures provide “even performance across the drive”. In actual fact, PM reduces performance everywhere by throttling performance to half-speed or worse. One can just as well partition drive(s) and maintain high performance across a desired capacity.

The green/blue lines below show that the claim for consistent performance doesn’t entirely hold up under scrutiny: observe the 0% mark. Results obtained with DiskTester.

Performance penalty using Port Multiplication
Horizontal axis represents % across the drive, vertical is MB/sec
(4-drive stripe using Western Digital 1TB RE3)

The graph above shows the huge performance hit from port multiplication. Striped RAID performance with as few as two fast drives will suffer from a port-multiplied enclosure. Your choice for really fast eSATA is to use two or more PM enclosures, or to use a non-PM enclosure, the latter always providing superior performance with the same number of drives.

Always test your setup with DiskTester.