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I set up my Raspberry Pi as a Time Machine, as documented here.

At the moment, the Raspberry Pi is connected to my MacBook Pro using a direct Ethernet cable. Also, an external hard drive (laptop drive) is connected to the Raspberry Pi using the USB port. However, backups are pretty slow. Activity Monitor claims that the Network is transferring a very steady 5 Mb/s, where my Time Capsule is transferring up to 8 Mb/s with a lot of fluctuation.

The Raspberry Pi self-reports (top) that its CPU is only half-used, with about equal parts afpd, usb-storage and jbd2/sda1-8. Thus, I think that the processing power of the Raspberry Pi does not seem to be the problem here.

To me, this looks like there is some kind of bottleneck that maxes out at 5 Mb/s thus potentially having my backups run at less than their potential speed. To the best of my knowledge, this might be the afp-daemon, the usb-bus or the external hard drive.

So, my question is, how could I identify the true culprit and what can I do about it?

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Raspberry Pi network card is connected via USB bus, so it's throughput is limited. 10MB/s [usb to disk + lan to usb] + 1-2MB/s for extra upstream data ~= 11.5MB/s which is fairly good result for device like RPi.

p.s. I think you talking about MB/s (megabytes per seconds) - not Mb/s (megabits per second).

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