Question: I have a 120GB SSD as the boot drive in my current PBX. This normally means my Clonezilla backup must be restored on a disk at least as big.
Is there a different way to virtualize the current machine into a 30GB (or whatever size) Proxmox container?
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Assuming that your system partition is ext3 or ext4, the easiest and SAFEST way to shrink the size of your system partition is to use gparted while your machine is started from a live distro CD/DVD/FlashDisk.
I recommend LinuxMint for that task. Start your machine with the live OS. The system partition which holds your PBX will be untouched, unmounted, etc.
While running LinuxMint in live mode (which you do not install), you bring up gparted (partition manager) GUI.
It will show you your disks and partitions. Your PBX partition will probably be /dev/sda1 or /dev/nvme0p1.
Once you found it, through the gparted GUI resize it to whatever minimum (plus some margin) you want.
All that you have to do is to right click on the partition and select resize. Once the resize window is up, you can drag the right hand side limit to the left in order to shrink or to the right in order to enlarge. Do not touch the left hand side handle at all (which would cause all the data in the partition to move, and you do not want that as it is very time consuming). If you can't move the limit/handle, either the partition is in use or it is mounted, or the drive is in read only mode. Gparted won't let you to shrink the partition to less than the amount of data already present in the partition. But give yourself some additional empty space. Don't push it too low, otherwise you won't be able to start it later (without resizing it up), especially if you use a dynamic swap file.
Before shrinking gparted will check the integrity of the partition and size down only if it is feasible. It takes a bit of time, but it works beautifully. And it's the safest way you can d it!
Then do your backup, whatever.
If you need to bring it back to the original size, assuming that you didn't use the saved space for something else (like a new partition), you can do it the same way, except you expand it instead of shrinking. You can expand as long as the available space is adjacent to the resized partition.
gparted can't resize (and you shouldn't either by manual resizing methods) a partition which is mounted and/or is in use. Thus the necessity to start your system with a live distro.
When you handle gparted, just make sure that you are sure which partition you need to resize. Even if you have only one drive, it could be sda1 or sda2 (BIOS boot or EFI boot). So, check and double check!