Finding read throughput using "dd"

If you are trying to find the read throughput of a device (like an SD card), you can do the following:

1. If possible unmount the device but keep it connected. Let's say you are looking at an SD card at sdd. (Don't worry about the volumes)
2. Run the following:
sudo dd if=/dev/sdd of=/dev/null bs=50M count=1

This will print the read throughput, something like this:
1+0 records in
1+0 records out
52428800 bytes (52 MB) copied, 2.62946 s, 19.9 MB/s

Now, if the device is mounted, you might see a variability in the throughput because of file system page caching between runs. To avoid that, free the page caches using:
sync; echo 1 | sudo tee /proc/sys/vm/drop_caches

NOTE: To free deentries and inodes the following can be used:
sync; echo 2 | sudo tee /proc/sys/vm/drop_caches

If you want to free up everything (page caches, deentries and inodes), do the following:
sync; echo 3 | sudo tee /proc/sys/vm/drop_caches

Use tee to set values in proc or sysfs

There are times when you can't do a "echo something > /proc/somewhere", especially when you got to use "sudo".

Using tee helps:

echo value | sudo tee /proc/somewhere