[SOLVED] Does converting qcow2 without -c remove compression?

[Terrum]

I have a qcow2 disk image that I compressed with "qemu-img convert -c". But I understand that this can inflict performance issues when running a compressed qcow2 disk image.

Thankfully I only plan to use the compressed qcow2 disk images as templates. So when I clone the compressed qcow2 template from the web UI, will it clone the disk image without compression and restore normal qcow2 performance?

Thank you for any clarifications!

 

[dylanw]

From what I understand, qcow2 compression is a "one time" process, where data is compressed at that time, but any further data written is uncompressed, so after cloning a template, all data written will be uncompressed.
The performance issue that you're referring to relates only to the time it takes to compress an image.

See:
https://www.jamescoyle.net/how-to/1810-qcow2-disk-images-and-performance
https://docs.fedoraproject.org/en-U...alization-Tips_and_tricks-Using_qemu_img.html

 

[Terrum]

Thanks for your clarification, however

The performance issue that you're referring to relates only to the time it takes to compress an image.

According to one of the sources you provided https://www.jamescoyle.net/how-to/1810-qcow2-disk-images-and-performance it mentions that any compressed data will need to be decompressed as soon as it is accessed, so I assume this would degrade performance.

I just want to clarify whether I am understanding this correctly, and if I will receive any performance issues during the live use of the compressed disk image after it has been compressed.

Thanks once again!

 

[dylanw]

Ah sorry about that..

So it seems the answer depends on how VMs will be cloned from the template. If you are creating linked clones, because the same underlying data is being referred to, you will have to pay the performance price of decompressing the data on read. However, if you create a full clone, the image will be decompressed while cloning.

 

[Terrum]

No worries. In that case, if I have a compressed 'qcow2' and decide to full clone it to a 'raw', this should decompress all of the existing data and ensure any potential performance issues caused by compression to be void?

Many thanks!

 

[dylanw]

Yes. I just double checked on my machine to confirm

Happy to help!

 

[Terrum]

Thank you so much!

I can therefore mark this thread as solved now. Appreciate it very much