Hi all,
I am trying to shrink a qcow2 image without corrupting data. I already made sure, from inside the VM, that at the end of the disk there are ~8G worth of free sectors. The total disk size is 80G, so only the first ~72G are used as shown by fdisk:
```
Disk /dev/vda: 80 GiB, 85899345920 bytes, 167772160 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: EEB134A0-C938-46CA-9385-23A5F358AA64
Device Start End Sectors Size Type
/dev/vda1 2048 4095 2048 1M BIOS boot
/dev/vda2 4096 4198399 4194304 2G Linux filesystem
/dev/vda3 4198400 150999040 146800641 70G Linux filesystem
```
The system works fine in this state and my understanding is that there is nothing relevant after sector 150999040 (~72G into the disk) other than the backup GPT table.
So my next step was to actually resize the disk (to 75G, so there's still a lot of buffer if there are some relevant sectors after vda3 ends):
```
# qemu-img info Ubuntu2.img
image: Ubuntu2.img
file format: qcow2
virtual size: 80 GiB (85899345920 bytes)
disk size: 39.9 GiB
cluster_size: 65536
Format specific information:
compat: 1.1
lazy refcounts: false
refcount bits: 16
corrupt: false
# qemu-img resize --shrink Ubuntu2.img 75G
Image resized.
# qemu-img info Ubuntu2.img
image: Ubuntu2.img
file format: qcow2
virtual size: 75 GiB (80530636800 bytes)
disk size: 39.9 GiB
cluster_size: 65536
Format specific information:
compat: 1.1
lazy refcounts: false
refcount bits: 16
corrupt: false
```
This looked fine to me, and I proceeded to boot the system. Sadly, no partitions are recognized any longer and the boot to disk fails. With GParted Live I get the error "Invalid argument during seek for read on /dev/vda" (
https://i.imgur.com/c45TZj4.png), and if I ignore that and an issue with one of the GPT tables then it shows everything as unallocated space. So I think something has gone wrong during the resize, hasn't it?
Interestingly enough, I can increase the size of the disk to 80G again with `qemu-img`, and afterwards the system works perfectly fine again. I get the error `The backup GPT table is corrupt` when running `fdisk -l /dev/vda`, which makes sense, but it can easily be restored by `sgdisk -e /dev/vda`.
I would be happy for any suggestion about what I might be doing wrong or how to fix this.
Thank you,
Nicolai Spohrer
