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[Qemu-discuss] Info about compressed images

From: Gianni Costanzi
Subject: [Qemu-discuss] Info about compressed images
Date: Fri, 13 Jan 2017 18:57:53 +0100

I'm new here and I've started using QEMU when I've started playing with Unetlab/EVE Network simulator.

I have some questions about compression and images with a compressed backing file:

1) If I create a compressed qcow2 image A and then I create B with backing file A, will B contents be compressed? If no, is it possible to have compression enabled also on image B?

2) I have an uncompressed qcow2 image A and an image B created with backing file A. Then I convert image A into C enabling compression (with qemu-img convert -c). When you rebase an image, I've understood that qemu looks at every changed block from source base to destination base and stores the different blocks within the image you're rebasing. If I rebase B from A (uncompressed) to C (compressed) will qemu compare A blocks of data with uncompressed C blocks of data (thus not growing up B image) or will it find the whole image as changed and store lots of changes within image B?

3) Is there a command that I can use to see if an image has compression enabled? qemu-img info does not show that info

PS: Unetlab/Eve virtual environment uses qemu v2.4.0

Thank you in advance for any help, best regards
    Gianni Costanzi 

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