Out of curiosity: Why is virtual disk compacting not possible like in hyper-v?

[AlpsView]

In Hyper-V, if i want to shrink a vhdx runnig i.e. Ubuntu as guest os, i can shrink the virtual disk by first setting all unused blocks to 0 with sudo fstrim /, shut the vm down and on Powershell run Optimize-VHD virtualdisk.vhdx -Mode Full. Optimize-VHD obv. works by detecting all unused blocks (zeros) which is why fstrim is needed. I have done this a good number of times and it always worked fast and smooth.

Not knowing how this and how virtual disks are working in detail and with no intention for a A is better than B discussion, I'm wondering why this approach wouldn't work with proxmox or better said with the virtual disks we are using in proxmox. It would be a very nice feature to have, as the "gold standard" with cloning to a reduced size disk is just more work and not integrated into the proxmos UI. Again, I'm not complaining, just trying to understand if this is more a "we haven't tried it" or a "can't be done" or a "other things are more imporant" or "...." "issue".

 

[alexskysilk]

virt-sparsify /path/to/source.qcow2 --compress /path/to/output.qcow2

 

[AlpsView]

I understand, that's freeing up unused space of a dynamic size virtual disk. So that's basically what I described. I need to distinguish between compacting and reducing size. Not the same, even when compacting leads the same result in terms of reduced space needed, since a dynamic disk still can grow to its initial capacity if needed.

So compacting is not a problem, but reducing comes with hurdles and challenges?

 

[alexskysilk]

oh you mean reducing logical size? thats ALWAYS a fraught prospect. having this function integrated into vhdx is nice although I'm not certain it would work for any guest filesystem other then ntfs (not an expert in this in any case there is always clone to smaller as an available method.

 

[LnxBil]

In Hyper-V, if i want to shrink a vhdx runnig i.e. Ubuntu as guest os, i can shrink the virtual disk by first setting all unused blocks to 0 with sudo fstrim /, shut the vm down and on Powershell run Optimize-VHD virtualdisk.vhdx -Mode Full. Optimize-VHD obv. works by detecting all unused blocks (zeros) which is why fstrim is needed. I have done this a good number of times and it always worked fast and smooth.

This can be done in PVE too and depending on the storage backend (e.g. ZFS and to less optimized extend CEPH, LVM-thin), you don't need the Optimize-VHD run, just fstrim and everything else is done automatically.