The speed of a drive is measured in terms of how much data it can read or write in unit time. The dd command is a simple command line tool that can be used to read and write arbitrary blocks of data to a drive and measure the speed at which the data transfer took place.
In this post we shall use the dd command to test and read and write speed of usb and ssd drives using the dd command.
The data transfer speed does not depend solely on the drive, but also on the interface it is connected to. For example a usb 2.0 port has a maximum operational speed limit of 35 Mbytes/s, so even if you were to plug a high speed usb 3 pen drive into a usb 2 port, the speed would be capped to the lower limit.
The same applies to SSD. SSD connect via SATA ports which have different versions. Sata 2.0 has a maximum theoretical speed limit of 3Gbits/s which is roughly 375 Mbytes/s. Whereas Sata 3.0 supports twice that speed.
Test Method - The dd command
Mount the drive and navigate into it from the terminal. Then use the dd command to first write a file using fixed sized blocks. Then read the same file out using the same block site.
The general syntax of the dd command looks like this
dd if=path/to/input_file of=/path/to/output_file bs=block_size count=number_of_blocks
When writing to the drive, we simply read from /dev/zero which is a source of infinite useless bytes. And when read from the drive, we read the file written earlier and send it to /dev/null which is nowhere. In the whole process, dd keeps track of the speed with which the transfer takes place and reports it.
The SSD that we are using is a "Samsung Evo 120GB" ssd. It is a beginner level ssd that comes within a decent budget and is also my first SSD. It is also one of the best performing ssds, in the market.
In this test the ssd is connected to a sata 2.0 port.
Lets first write to the ssd
$ dd if=/dev/zero of=./largefile bs=1M count=1024 1024+0 records in 1024+0 records out 1073741824 bytes (1.1 GB) copied, 4.82364 s, 223 MB/s
Block size is actually quite large. You can try with smaller sizes like 64k or even 4k.
Now read back the same file. However, first clear the memory cache to ensure that the file is actually read from drive.
Run the following command to clear the memory cache
$ sudo sh -c "sync && echo 3 > /proc/sys/vm/drop_caches"
Now read the file
$ dd if=./largefile of=/dev/null bs=4k 165118+0 records in 165118+0 records out 676323328 bytes (676 MB) copied, 3.0114 s, 225 MB/s
The Arch Linux wiki has a page full of information about the read/write speed of various SSDs from different vendors like Intel, Samsung, Sandisk etc. Check it out at the following url.https://wiki.archlinux.org/index.php/SSD_Benchmarking
USB Pen drives
In this test we shall measure the read and write speed of ordinary usb/pen drives. The drives are plugged to standard usb 2 ports. The first one is a sony 4gb usb drive and the second is a strontium 16gb drive.
First plug the drive into the port and mount it, so that it is readable. Then navigate into the mount directory from the command line.
Sony 4GB USB 2.0 Drive
Sony 4GB - Write
In this test, the dd command is used to write 10,000 chunks of 8 Kbyte each to a single file on the drive.
# dd if=/dev/zero of=./largefile bs=8k count=10000 10000+0 records in 10000+0 records out 81920000 bytes (82 MB) copied, 11.0626 s, 7.4 MB/s
So the write speed is around 7.5 MBytes/s. This is a low figure.
Sony 4GB - Read
The same file is read back to test the read speed. Run the following command to clear the memory cache
$ sudo sh -c "sync && echo 3 > /proc/sys/vm/drop_caches"
Now read the file using the dd command
# dd if=./largefile of=/dev/null bs=8k 8000+0 records in 8000+0 records out 65536000 bytes (66 MB) copied, 2.65218 s, 24.7 MB/s
The read speed comes out around 25 Mbytes/s which is a more or less the standard for cheap usb drives.
USB 2.0 has a theoretical maximum signaling rate of 480 Mbits/s or 60 Mbytes/s. However due to various constraints the maximum throughput is restricted to around 280 Mbit/s or 35 Mbytes/s. Beyond this the actual speed achieved depends on the quality of the pen drives and other factors too.
And the above usb drive was plugged inside a USB 2.0 port and it achieved a read speed of 24.7 Mbytes/s which is not very bad. But the write speed lags much behind
Strontium USB 2.0 16GB Pen drive
Now lets do the same test with a Strontium 16gb drive. Strontium is another very cheapy brand, although usb drives are reliable.
# dd if=/dev/zero of=./largefile bs=64k count=1000 1000+0 records in 1000+0 records out 65536000 bytes (66 MB) copied, 8.3834 s, 7.8 MB/s
Strontium 16gb read speed
# sudo sh -c "sync && echo 3 > /proc/sys/vm/drop_caches" # dd if=./largefile of=/dev/null bs=8k 8000+0 records in 8000+0 records out 65536000 bytes (66 MB) copied, 2.90366 s, 22.6 MB/s
The read speed is lower than the Sony drive.
SanDisk Cruzer Blade 32GB USB Flash Drive
This is a popular cheap USB 2.0 drive available on amazon.in
Lets test its read and write speed.
Read Speed - CPU Case USB Port
Reading a simple iso file present on the drive using the dd command.
$ dd if=./ubuntu.iso of=/dev/null bs=8k 90355+1 records in 90355+1 records out 740195568 bytes (740 MB, 706 MiB) copied, 27.0821 s, 27.3 MB/s
The read speed is about 27.3 MB/s
Write Speed - CPU Case USB Port
Now lets test the write speed
$ dd if=~/ubuntu-16.04.5-desktop-i386.iso of=./ubuntu-16.04.5-desktop-i386.iso bs=1M 1587+1 records in 1587+1 records out 1664614400 bytes (1.7 GB, 1.6 GiB) copied, 374.269 s, 4.4 MB/s
The write speed is quite low at 4.4 MB/s.
Sandisk Ultra CZ48 USB 3.0 16 GB Pen Drive
Read speed - Motherboard USB Port
$ dd if=./backup.rar of=/dev/null bs=8k 51108+1 records in 51108+1 records out 418678907 bytes (419 MB, 399 MiB) copied, 4.82621 s, 86.8 MB/s
This one has a decent read speed.
Write speed - Motherboard USB Port
Lets copy the same ubuntu iso file from disk to the usb drive
$ dd if=~/ubuntu-16.04.5-desktop-i386.iso of=./ubuntu-16.04.5-desktop-i386.iso bs=1M 1587+1 records in 1587+1 records out 1664614400 bytes (1.7 GB, 1.6 GiB) copied, 128.624 s, 12.9 MB/s
The write speed this time is better than the earlier Sandisk USB 2.0 drive.
Motherboard USB Port vs CPU case USB Port - Big Difference!
The speed also depends on whether you connect the USB drive to the front ports on your cpu case or directly on the motherboard usb ports. As you might be able to guess already, the speed on motherboard usb ports is significantly higher.
Front of CPU Case - Sony USB 3.1 Gen 16GB - Read Speed
$ dd if=./ubuntu.iso of=/dev/null bs=8k 90355+1 records in 90355+1 records out 740195568 bytes (740 MB, 706 MiB) copied, 17.6587 s, 41.9 MB/s
Motherboard USB Port - Sony USB 3.1 Gen 16GB - Read Speed
$ dd if=./ubuntu.iso of=/dev/null bs=8k 90355+1 records in 90355+1 records out 740195568 bytes (740 MB, 706 MiB) copied, 6.11666 s, 121 MB/s
So the motherboard usb port speed is almost 3X higher than the cpu case usb ports. So if you are planning to do heavy data transfer, then it would be a better idea to make the extra effort of connecting the usb drive to the back of your CPU.
Motherboard USB Port - Sony USB 3.1 Gen 16GB - Write Speed
$ dd if=~/ubuntu-16.04.5-desktop-i386.iso of=./ubuntu-16.04.5-desktop-i386.iso bs=1M 1587+1 records in 1587+1 records out 1664614400 bytes (1.7 GB, 1.6 GiB) copied, 176.581 s, 9.4 MB/s
The write speed of this Sony USB 3.1 drive is lower than the Sandisk Ultra USB 3.0 Drive. Both of them were tested on motherboard usb ports.