There is a major problem with the Apple AirPort Extreme involving its "AirDisk" feature. I have the "older" 100BaseT model to which I have attached a USB hard drive to share among my various Macs. There have been many postings on the Apple Discussion Boards about poor disk performance when the Airport Extreme is used to share disks. While some users report slow throughput, others claim to have no issues at all.
I have confirmed, through testing, that some USB disks do indeed exhibit very poor write throughput when compared to other USB disks. This could have a serious impact for anyone using a USB disk with an AirPort Extreme, especially when used for its intended purpose: to do unobtrusive backups.
This problem was first brought to my attention when I purchased an Other World Computing (OWC) Mercury Elite Pro "Quad Interface" enclosure. In it I placed a Seagate Barracuda 3.5" 400GB SATA drive. This enclosure, which sports four types of interfaces (USB 2.0, FW400, FW800, and eSATA) for attaching to a computer, performed admirably over FW800 when connected directly to my PowerMac G5 tower. But when I connected it to my Airport Extreme, throughput suffered. Writing to the drive was especially bad.
The drive enclosure is manufactured by Newer Technologies. Many emails with their technical support division turned up no solutions. In fact, Newer's tech support denied that there was any kind of performance issue.
In desperation, I connected up an old "generic" USB enclosure with an even older 20GB IDE hard drive. To my surprise, throughput was quite good. Clearly, there is some bad interaction between the AirPort Extreme, the USB enclosure and its internal driver circuitry, and the hard drive.
Ultimately, I wanted a nicer enclosure that would fit under my AirPort Extreme. I settled on the miniStack v3. Like the Mercury Elite Pro, the miniStack v3 is also a quad interface enclosure. It, too, is manufactured by Newer Technologies. I also purchased a 500GB Hitachi SATA drive. After hooking everything up, the results were the same: write performance through the Airport Extreme was poor.
Finally, at wit's end, I decided to settle this once and for all and run some tests. I suspect that there's something about the Newer Technologies driver circuitry that is reducing throughput. I wanted to pit the miniStack against the cheapest USB enclosure I could fine. In the tests, I attempted to eliminate as many variables as possible. The only variable would be the USB enclosure. I also tested a direct USB connection to the Mac to obtain baseline figures.
Newer Technologies miniStack v3 with quad interfaces ($120, from OWC);
AverTech SATA HDD Enclosure with only a USB 2.0 interface ($35, at Fry's).
One hard drive:
Seagate Barracuda 7200.10 3.5" 400GB SATA hard drive.
Two connection types:
Directly connected to the Mac via USB cable;
Via the AirPort Extreme, using 100BaseT Fast Ethernet (not wireless). AirPort Extreme running v 7.1.1 firmware.
I ran eight tests. Four were "read" tests and four were "write" tests. All tests used the same 600MB mix of files and folders. The files ranged in sizes from under 1K to over 200MB. Before each write test, the drive was reformatted as HFS+ with journaling using the Apple Disk Utility.
The results are as follows (click for larger version):
In all tests, the miniStack is no faster than the "cheapie" USB enclosure. In fact, it's always slower. But most alarming is the write throughput via the AirPort Extreme. The miniStack is takes almost twice as long!
Also worth noting is the noise the drive makes when performing the tests. In seven of the eight tests (all except the slow write test noted above), the drive is nearly silent. All I can hear is the motor and a very quiet clicking as the heads move back and forth. In contrast, during the AirPort Extreme write test using the miniStack, the drive is very loud. The heads can be heard noisily clacking back and forth. Something different is clearly happening with the miniStack.
The OWC Mercury Elite Pro "Quad Interface" and the Newer Technologies miniStack v3 both behave nearly identically. I suspect that the internals of both enclosures are pretty much the same, as they are both made by the same manufacturer (Newer Tech) and both are quad interface devices. Both enclosure exhibit very slow write throughput and create lots of noise when connected to my Mac via the AirPort Extreme.
The AverTech USB enclosure (the cheapest thing I could find at my local Fry's) is identical in performance to the miniStack except when writing files via the AirPort Extreme. In that test, it is much faster and much quieter.
I suspect that there is some kind of bad interaction between the Apple AirPort Extreme and these "multi interface" enclosures. A no-name USB-only enclosure soundly beats a name brand multi-interface one. Someone, either Apple or the USB drive enclosure manufacturers, needs to explain these dismal throughput numbers.
I've learned one thing for sure: spending more -- a lot more -- on an enclosure doesn't necessarily get you better quality or higher speed.
Update (7:35 pm)
Just for grins, I included two more enclosure and drive combinations in my tests. All told, they are:
Newer Technologies miniStack v3 with Seagate 400GB SATA drive
AverTech "generic" USB with Seagate 400GB SATA drive
CompUSA "generic" USB with Quantum 20GB IDE drive
OWC Mercury Elite Pro Quad with Seagate 400GB SATA drive
Results (click for larger version):
Interestingly, the old Quantum IDE drive fared better in the AirPort write test than any of the others! I can't account for that, but I do know why its read speeed over direct USB is slower: it has a longer rotational latency and therefore takes longer to access data.
The whole point of the graph is to show that the Seagate SATA drive performs poorly in the Airport write test whether its in the miniStack or the Mercury Elite. Something is definitely wrong with this picture.
Update update (Aug. 21, 1:15 AM)
A reader suggested turning off HFS+ journaling on the external drive. I did so and clocked the fastest AirPort write time yet: 125 seconds for the 600MB of files. Throughput rises to an astonishing 4.8 Mbytes/sec! USB 2.0's maximum practical throughput is around 30 MBytes/sec. Fast ethernet can pump data at approximately 12 Mbytes/sec. Who knows what the internal disk transfer speed is of the Seagate drive, as Seagate does not publish that spec. I would expect performance of the overall system to be 6-9 Mbytes/sec.
Still, this points to a problem with the AirPort firmware: writing to HFS+ journaled multi-interface external disks is extremely slow. HFS+ with journaling is the default for newly-formatted disks, so there should be no reason why an ordinary user would need to turn it off.
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