How to Use ‘dd’ to Measure the Speed of Data Reads/Writes on Storage Devices in Linux

How to Use DD Utility for Storage I_O Performance Tests

In Linux, ‘dd’ is a versatile command-line utility that’s been used for decades for a variety of tasks, from copying and converting files to creating bootable USB drives. One of its lesser-known but incredibly useful applications is to measure the speed of data reads and writes on storage devices. This is crucial for webmasters and system administrators who need to ensure optimal performance of their storage devices, especially when hosting websites or applications.

In this guide, we’ll show you how to use ‘dd’ to conduct storage I/O performance tests on a Linux machine.


  • A Linux machine (any distribution will suffice).
  • Basic knowledge of the command line.
  • Root or sudo access to the machine.

Let’s get started.

Step 1. Open the Terminal

Access the terminal on your Linux machine. This can be done by searching for “terminal” in the application menu or by using the keyboard shortcut Ctrl + Alt + T.

Step 2. Testing Write Speed

To test the write speed of your storage device, use the following command:

sudo dd if=/dev/zero of=/tmp/test1.img bs=1G count=1 oflag=dsync

This command writes a 1GB file named ‘test1.img’ to the /tmp directory. The ‘oflag=dsync’ option ensures that ‘dd’ writes data in a synchronous manner, providing a more accurate representation of write speeds.

Step 3. Testing Read Speed

After testing the write speed, you can test the read speed using:

sudo dd if=/tmp/test1.img of=/dev/null bs=1G count=1 iflag=dsync

This command reads the previously written ‘test1.img’ file and sends the data to /dev/null, effectively discarding it. The ‘iflag=dsync’ option ensures synchronous reading.

Step 4. Interpreting the Results

After executing the commands, ‘dd’ will display the amount of data transferred, the time taken, and the speed. For instance:

1+0 records in
1+0 records out
1073741824 bytes (1.1 GB, 1.0 GiB) copied, 10.1234 s, 105 MB/s

In this example, the speed is 105 MB/s.

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Step 5. Cleanup

After testing, it’s a good practice to remove the test file to free up space:

sudo rm /tmp/test1.img

Additional ‘dd’ Usage Examples for I/O Performance Testing

1. Testing with Different Block Sizes

The block size (bs) parameter determines the size of the chunks of data that ‘dd’ reads and writes at a time. By varying this size, you can simulate different types of I/O operations:

sudo dd if=/dev/zero of=/tmp/test2.img bs=512 count=2000 oflag=dsync

This command writes a 1MB file using a block size of 512 bytes, simulating smaller, more frequent write operations.

2. Testing Direct I/O

Using the oflag=direct and iflag=direct options, you can bypass the filesystem cache, providing a more accurate representation of raw disk performance:

sudo dd if=/dev/zero of=/tmp/test3.img bs=1G count=1 oflag=dsync, direct

3. Testing Multiple Files Simultaneously

To simulate concurrent I/O operations, you can run multiple ‘dd’ commands in parallel:

sudo dd if=/dev/zero of=/tmp/test4a.img bs=1G count=1 oflag=dsync &
sudo dd if=/dev/zero of=/tmp/test4b.img bs=1G count=1 oflag=dsync &

This will start two ‘dd’ processes simultaneously, each writing to a different file.

4. Testing Read Speed of a Specific File

If you have a particular file you’d like to test the read speed of:

sudo dd if=/path/to/your/file of=/dev/null bs=1G iflag=dsync

Replace /path/to/your/file with the actual path to your file.

5. Writing to a Specific Partition or Disk

You can also write directly to a partition or disk (be very careful with this as it will overwrite data):

sudo dd if=/dev/zero of=/dev/sdX bs=1G count=1 oflag=dsync

Replace /dev/sdX with your actual device name (e.g., /dev/sda).

6. Creating a File with Random Data

To test write speed using random data (which might be slower due to the randomness):

sudo dd if=/dev/urandom of=/tmp/test5.img bs=1G count=1 oflag=dsync

The ‘dd’ command is a powerful tool in the Linux arsenal, especially for I/O performance testing. By understanding and varying its parameters, you can simulate a wide range of I/O scenarios and gather valuable insights into your storage device’s capabilities. Always remember to exercise caution when using ‘dd’, especially when writing directly to partitions or disks, to avoid data loss.

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  1. Why is ‘dd’ preferred for I/O performance testing?

    ‘dd’ is a built-in Linux utility, making it readily available on almost all Linux distributions. Its simplicity and versatility make it a preferred choice for quick and accurate I/O performance tests.

  2. Can ‘dd’ be used on mounted partitions?

    Yes, ‘dd’ can be used on mounted partitions. However, it’s crucial to ensure that the test file does not fill up the partition, as this can lead to system instability.

  3. Is there any risk in using ‘dd’ for performance testing?

    While ‘dd’ is safe for performance testing, it’s a powerful tool that can overwrite data if used incorrectly. Always double-check the input and output paths and ensure you’re writing to or reading from the intended locations.

  4. How can I improve my storage device’s performance?

    Improving storage device performance can involve several strategies, including defragmenting the device (for HDDs), ensuring the filesystem is optimized, upgrading to faster storage solutions like SSDs, or using RAID configurations for enhanced speed and redundancy.

  5. Why might I get different results on repeated tests?

    Several factors can lead to varying results, including other processes accessing the storage device during testing, the state of the device’s cache, or the overall system load. It’s advisable to run the test multiple times and consider an average for more consistent results.

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Commands Mentioned

  • sudo dd if=/dev/zero of=/tmp/test1.img bs=1G count=1 oflag=dsync – Tests the write speed of the storage device.
  • sudo dd if=/tmp/test1.img of=/dev/null bs=1G count=1 iflag=dsync – Tests the read speed of the storage device.
  • sudo rm /tmp/test1.img – Removes the test file created during the performance test.


Measuring the I/O performance of storage devices is crucial for ensuring optimal system performance, especially in environments where data transfer rates are critical, such as web hosting. Whether you’re running a shared hosting environment, a VPS server, a dedicated server, or even a cloud hosting setup, the speed at which data can be read from and written to storage devices can significantly impact the user experience and overall system efficiency.

The ‘dd’ command, while simple, offers a straightforward method to gauge this performance. However, it’s essential to approach these tests with caution. As with any tool, understanding its capabilities and potential pitfalls is crucial. When used correctly, ‘dd’ can provide valuable insights into your storage device’s performance, allowing you to make informed decisions about potential upgrades or optimizations.

Furthermore, while ‘dd’ is an excellent tool for quick performance checks, those looking for more detailed insights or continuous monitoring might consider specialized I/O benchmarking and monitoring tools available for Linux. These tools can offer more granular data, various testing scenarios, and even graphical interfaces for easier interpretation of results.

In web hosting and server management, staying informed about your hardware’s performance is not just beneficial—it’s essential. Regularly conducting I/O performance tests ensures that you’re always aware of your system’s capabilities, allowing you to address potential issues proactively and ensure that your applications and websites run smoothly.


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