AirPort Disk - Throughput Testing

Note: An "AirPort Disk" is what Apple calls a shared external USB drive attached to the USB port of either an AirPort Extreme base station (AEBS) or a Time Capsule (TC).

Do you have a super fast network, but it still feels like it takes forever to copy or retrieve a file from an AirPort Disk? It may be possible that your network is not optimized to provide the highest throughput that it can offer ... but how do you get a objective measurement to compare against?

There are a few things to take into consideration, namely:

1. The throughput between your computer and the AEBS or TC, and
2. The throughput between your computer and the AirPort Disk.

For comparison I did three simple test scenarios for my network.

• Scenario #1: Late 2014 Intel Mac mini (macOS Sierra 10.12.6) > 1st generation 802.11ac TC (7.7.8)
• Scenario #2: Mac mini > TC > Western Digital My Passport USB 1 TB HDD
• Scenario #3: Mac mini > Synology DS916+ NAS (DSM 6.1.3)

The basic network path between them are:

• Scenario #1: Mac mini > Ethernet Switch > Cisco Router > Ethernet Switch > TC
• Scenario #2: Mac mini > Ethernet Switch > Cisco Router > Ethernet Switch > TC > USB > WD Passport
• Scenario #3: Mac mini> Ethernet Switch > Cisco Router > Ethernet Switch > DS916+
• Note: All network connections are via Gigabit Ethernet, except the connection between the TC and the HDD which is via v2.0 USB. Substituting wireless connections, at any junction, will result in an overall reduction in throughput. As you can imagine, you can have literally thousands of other network configurations and each will be unique for their overall throughput characteristics.

Measuring network throughput between the Mac mini and the TC or 916+

Method 1 - Using IPNetTunerX

I first measured the data throughput from the Mac mini to the TC by using IPNetTunerX. I used iPNetTunerX's Link Rate test tool. I did five runs and the average throughput was found to be 900.7 Mbps (112.6 MBps). Not bad as this is near the maximum possible bandwidth of a Gigabit Ethernet connection through three switches. I repeated the same test runs with the DS916+ and found even better results: 1037.1 Mbps (129.6 MBps)

Measuring throughput between the Mac mini and the AirPort Disk

Method 1 - Using macOS Sierra's Activity Monitor

To measure throughput to the AirPort Disk, I first created a 1 GB disk image file using the Disk Utility. For reference, I first copied this file to the root folder on the DS916+. Then, I did the same for the TC. Finally, I copied the same file to the AirPort Disk attached to the TC's USB port. To get the throughput reading I used Sierra's built-in Activity Monitor. I did five runs of copying this file and the average throughput was found to be:

• DS916+:
  • Read: 866.4 Mbps (108.2 MBps)
  • Write: 859.2 Mbps (107.4 MBps)
• Time Capsule:
  • Read: 418.4 Mbps (52.3 MBps)
  • Write: 313.6 Mbps (39.2 MBps)
• AirPort Disk:
  • Read: 144 Mbps (18 MBps)
  • Write: 48 Mbps (4 MBps)

Method 2 - Using AJA System Test

A second way to run a throughput test to the AirPort Disk is to use AJA System Test. System Test will automatically create a temporary file and perform read/write tests, then delete the file when done. Again, I did five runs transferring a 1 GB file.

• DS916+:
  • Read: 867.2 Mbps (108.4 MBps)
  • Write: 862.4 Mbps (107.8 MBps)
• Time Capsule:
  • Read: 408.0 Mbps (51.0 MBps)
  • Write: 336.0 Mbps (42.0 MBps)
• AirPort Disk:
  • Read: 128 Mbps (16 MBps)
  • Write: 32 Mbps (4 MBps)

Method 3 - Using Disk Speed Test

For the final test runs I used Blackmagicdesign's Disk Speed Test. I did five runs transferring a 1 GB file.

• DS916+:
  • Read: 869.6 Mbps (108.7 MBps)
  • Write: 860.0 Mbps (107.5 MBps)
• Time Capsule:
  • Read: 411.2 Mbps (51.4 MBps)
  • Write: 351.2 Mbps (43.9 MBps)
• AirPort Disk:
  • Read: 122.4Mbps (15.3 MBps)
  • Write: 85.6 Mbps (10.7 MBps)

Conclusion

Taking the average of each of the above tests:

• DS916+
  • Read: 867.7 Mbps (108.4 MBps)
  • Write: 860.5 Mbps (107.5 MBps)
• TC
  • Read: 412.5 Mbps (51.5 MBps)
  • Write: 333.6 Mbps (41.7 MBps
• AirPort Disk
  • Read: 131.46 Mbps (16.4 MBps)
  • Write: 55.20 Mbps (6.9 MBps)

If you were expecting the AirPort Disk to be competitive performance-wise with a dedicated NAS, you will be highly disappointed. As I expected the throughput of the AirPort Disk would overall be significantly lower than that of the internal hard drive in the Time Capsule, let alone to a true NAS, due to the limits of TC's processor & USB 2.0 interface, disc rotational speeds, and disc interface being negatively additive. All in all, it took around a minute+ to transfer a 1 GB file. The AirPort Disk in my case would not be suited for any application that required quick access to either read or write data to this network drive. For example, streaming a HD video file. However, it should perform adequately for periodic access to files that may be stored on it.

For other comparisons, let's look at some other baseline test results using Disk Speed Test:

• Test configuration #1: Late 2014 Mac mini, internal Macintosh HD (Fusion Drive)

  Results: Read: 696.8 / Write: 381.2 Mbps

• Test configuration #2: Late 2014 Mac mini, 2GB WD HDD in NewerTech MiniStack enclosure directly connected to Mac mini by USB 2.0

  Results: Read: 101.6 / Write: 97.5 Mbps

• Test configuration #3: Late 2014 Mac mini, 2x 3GB WD HDD in WD My Drive PRO enclosure directly connected to Mac mini by Thunderbolt 2.

  Results: Read: 306.5 / Write: 251.4 Mbps

You can perform similar tests to determine if the issue is the connection to the AEBS/TC or to the external USB HDD (AirPort Disk) that you are using. These two values should not be off more than 60-80%. If they are, I would suspect the USB HDD. If both are extremely low, then it would indicate the connection to the AEBS/TC will require further troubleshooting.