How to Use the Command 'virt-sparsify' (with Examples)

The virt-sparsify command is a powerful tool designed to make virtual machine drive images thin-provisioned. This means it can reduce the physical space that a virtual disk consumes by converting it into a sparse format, essentially cutting down on the unused portions of the disk. However, a critical caveat is that this command should only be used on virtual machines that are offline; otherwise, there is a risk of data corruption. For more detailed information, you can refer to the official documentation at libguestfs.org .

Use Case 1: Create a Sparsified Compressed Image without Snapshots from an Unsparsified One

Code:

virt-sparsify --compress path/to/image.qcow2 path/to/image_new.qcow2

Motivation:

The primary motivation behind this use case is to reduce the storage footprint of a virtual machine image. Often, virtual machine images contain a lot of unused or redundant space, especially if the disks have been resized or a lot of files have been deleted over time. By using virt-sparsify, one can not only reclaim this unused space but also compress the resulting image, which makes it even smaller. This is particularly useful for archiving purposes or when transferring images over a network, as it results in faster transfers due to the reduced file size.

Explanation:

• virt-sparsify: This is the command used to perform the sparsifying operation. It is part of the libguestfs suite, which deals with handling virtual machine disk images.
• --compress: This option tells the command to compress the resulting image file. When used, the new image will be both sparsified and compressed, leading to significant savings in terms of disk space.
• path/to/image.qcow2: This is the source image file, the unsparsified original that you wish to reduce in size.
• path/to/image_new.qcow2: This is the destination image file, where the newly sparsified and compressed image will be saved.

Example Output:

Running the above command will yield a smaller, compressed version of the original .qcow2 image. For instance, if the original was 10 GB but only contained 4 GB of actual data, the output file could be substantially smaller, perhaps around 4.2 GB, depending on compressibility.

Use Case 2: Sparsify an Image In-Place

Code:

virt-sparsify --in-place path/to/image.img

Motivation:

There are scenarios where it’s beneficial to optimize disk space without creating a new file. This is where the in-place sparsification becomes incredibly useful. Instead of creating a separate, sparsified image, this approach modifies the original while maintaining the same file properties (such as name and location). This is particularly advantageous when dealing with large virtual disk files in environments with limited storage space, as it immediately reclaims unused space without the need for additional storage during processing.

Explanation:

• virt-sparsify: The core command that performs the sparsification.
• --in-place: This option specifies that the sparsification should occur directly on the original image file. No new file is created, and the existing file is reduced in size.
• path/to/image.img: This is the path to the original image file that will be sparsified in-place.

Example Output:

After the command is executed, the original .img file will have reduced size, reflecting only the actual data in use. There will be no additional files generated, and the file name will remain unchanged. For example, a 20 GB image file with only 8 GB of actual data could be reduced to slightly over 8 GB once unnecessary space is removed.

Conclusion:

The virt-sparsify command is a valuable tool for managing and optimizing virtual machine disk images. By either creating new sparsified and compressed images or modifying the original images in-place, users can ensure efficient use of storage resources. Whether for archiving, transferring, or simply managing storage on local systems, understanding and utilizing virt-sparsify can result in significant savings of both space and time.