I have a USB hard drive of 32 GB, I want to know the time it takes to copy 1 GB of data from the USB drive. I tried using dd if=/dev/sdb1 of=/dev/null

The output is this

dd if=/dev/sdb1 of=/dev/null bs=1G
 28+1 records in
 28+1 records out
 30751983616 bytes (31 GB) copied, 331.297 s, 92.8 MB/s

is the approach used by me is correct or am I doing it wrong. I cannot use other tool, I have to use dd command only.


Your command is incorrect, you tell dd to read by 1 GB chunks, but an unlimited number of times. To only measure the time spent to read 1 GB, you need to tell dd to read only one block:

dd if=/dev/sdb1 of=/dev/null bs=1G count=1

Note that buffering will make that command run faster if the device has previously be read.

  • Can we make block size upto 1TB? – Bidyut Nov 18 '16 at 7:19
  • 1
    @Bidyut That depends on your dd implementation, GNU dd accepts even T, P, E, Z, and Y. – jlliagre Nov 18 '16 at 7:23
  • @Bidyut but a portable script should only use the k suffix as this is the only one supported by POSIX. – jlliagre Nov 18 '16 at 7:30
  • @jilliagre I need to do it for 3 TB file system too, and I need to find the amount of time it takes to copy 1 TB of file. – Bidyut Nov 18 '16 at 7:43
  • Your partition size is 30 GB, it cannot hold a 1 TB file system. – jlliagre Nov 18 '16 at 7:46

First, as @jiliagre said, you're measuring the transfer time of the whole partition (31 Gb), not 1 Gb. This is shown by the last line of your output:

30751983616 bytes (31 GB) copied, 331.297 s, 92.8 MB/s

Second, since you're writing to /dev/null and hence blackholing the data, it only gives you the read times for your USB drive.

The real I/O transfer time will be bound by the write speed capacity of the destination device, and most probably will be slower.

Furthermore, it depends on which data you're copying. 1 Gb of small files is copied slower than a single 1-Gb file, and takes up more space in the device due to the allocation file size.

  • Can we make block size upto 1TB?? – Bidyut Nov 18 '16 at 7:19
  • I hear its best to lower the block size to match the device you are reading from and increase the count number. – jc__ Nov 18 '16 at 19:15

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