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I have been measuring my primary disk I/O with the following command :

dd if=/dev/zero bs=2048k of=/Volumes/Toshiba/tstfile count=1024 2>&1 | grep > sec | awk '{print $1 / 1024 / 1024 / $5, "MB/sec" }'

This test is obviously for a write operation, read can easily be measured as well.

Both read and write give a speed of about 28MB/s which seems pretty poor, despite the age of my mac mini (2GHz core 2 duo).

Furthermore, a 1TB usb 3 drive from last year gives the same performance, below 30MB/s always.

I have tried changing the file size from 1MB to 500MB, results are consistent.

Since both drives behave the same, i suspect a bottleneck somewhere else in the hardware or the software.

What are the suggestions to investigate further?

Thanks Nick

  • Plugging a USB 3 external drive into a USB 2 port (which your Mac Mini has) will never give you more than USB 2 speed. The theoretical absolute maximum throughput you can get from USB 2 is 480Mbps (60MB/sec), and 30MB/sec sounds a lot more realistic because of the overheads of the USB protocols. – Scott Earle Jul 18 '14 at 3:41
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My experience with dd is that as well as the file size, the block size makes a huge difference. Try playing with the bs parameter (try 128 - 10240) and see what sort of a difference it makes.

Also - a sustained 28MB/sec write speed is not all that bad for a 5-7 year-old computer. (Although if you multiply by 8 to get the speed in bits per second, you will see that it's 224mbps - around 15% of a SATA-I bus.)

Things to consider include:

  • Is the HDD a 5400rpm or 7200rpm model? For sustained larger file writes (as opposed to many small files), it makes a difference.
  • Not all HDDs are created equal, with some models being faster than others. Find out what model yours is and search a bit to find out how they usually fare.
  • Have you considered upgrading to an SSD? This will make a huge difference, even if the interface on the computer is only SATA-I (quite likely), as a decent SSD should be able to saturate the SATA bus.

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