shrinking qcow2 image virtual size

[skraw]

Hello all,

I have to move around some physical boxes and virtualise them later on. So I came across this question about qcow2 images and cannot find a conveniant solution for it.
Lets assume having a qcow2 disk image with a big virtual size and a relatively small partition size inside. How can I shrink the image to contain only this/these partition(s)? The problem with qemu-img resize is that you have to give it exact resize numbers. But since I want to make the sizes fitting exactly I would have to "calculate for the byte". Isn't there some way to tell a resize tool "shrink the unused space of a disk at the end" ?
--
Regards

 

[e.fastermann]

Hello skraw,

there is no built-in “auto shrink to used partitions” command in QEMU / Proxmox VE because disk images are treated as raw block devices and QEMU does not know which part of the disk is unused. You should be able to script this, but I currently don't have the time to look into this, sorry.

Best,
Erik

 

[EllerholdAG]

Is this even a real problem with thin provisioning?

 

[_gabriel]

alternative less error prone : create new vdisk at desired size
then clone like baremetal

 

[skraw]

Is this even a real problem with thin provisioning?

sometimes the real problem hides behind a simple question. In the meantime I found out that qemu-img resize does not work but shoots the disk resized, no matter how few bytes you shrink it. How about that?

 

[news]

Then use a ZFS Dataset and import your disk, with compression it will compress the data "on the fly".
I got a Win7 Clone of a 500 GB Disk. It use now 68 GB disk space.

 

[skraw]

Ok, i found another thread with quite the same problem showing up:

S

[SOLVED] Thread 'Trying to shrink qcow2 image to make space for another one on the proxmox host (image gets corrupted after resize)'

Dec 23, 2022

I have an issue here (the purple screenshot is from when I am booted in an ubuntu live iso) and I am unable to fix it but as you can see the device /dev/sda is 414G (the dark black is in proxmox terminal before converting the qcow2 image to raw for resize), but I want to get rid of the unallocated ~142G (shown in the gparted screenshot) and take it down to around 260-270G. It's a qcow2 image and whenever I convert it to raw and resize, the data becomes unreadable.

-proxmox terminal screenshot (host)-


-ubuntu live iso terminal (guest)-


-gparted...

• successtheman
• Replies: 17
• Forum: Proxmox VE: Installation and configuration

Only he tries to enlarge the disk.
With the hints he gave I found that qemu-img resize simply destroys the partition table, his partly, mine completely. And thats about the difference.
So if you save the table with

sfdisk --dump /dev/nbd0 > disk.dump
(I use this to acess the file as block device),

then

qemu-img resize --shrink blabla.qcow2 <somesize>

and then restore the partition table from the dump with

sfdisk --no-reread --no-tell-kernel -f --wipe never /dev/nbd0 < disk.dump

you win. if you are interested look at the partition table after the resize with sfdisk and find it completely destroyed
Contrary to the above thread here no lvm or complex other stuff is involved. I'd say qemu-img resize is really a bad implementation of what it should do.