VM initial HD configuration

[manolis11]

Hello,
i'm new on ProxMox and i have a question to ask you.

I 'm using a small test server with an internal SSD drive of 250GB with ProxMox 8.0.9
local Directory 88,25GB
LVM-thin 147,50 GB


Now I have create only one VM with fedora server 39 as an image and assign to the DISK during the initial setup 350 GB of disk size.
The VM is created without any issue.
lsblk indicates sda3/fedora-root with a size of 300G with mountpoint lvm /
using the command fallocate -l 300G demo.txt i have successfully created a txt file that occupies 300GB.
How it's possible? i have only 250GB of SSD?
where is the mistake?

When trying to do the same thing with a LCX Container with a disk size of 100GB for example i cannot create a file bigger than the disk size.
A message indicates that i dont have enough space.

could you please clarify this?

many thanks

 

[Folke]

Hi,
the fallocate command does not actually write any data (ignoring metadata of course). It just marks the blocks as reserved. If you would do something like dd if=/dev/zero of=testfile.dat bs=1G count=300 which actually writes data, you would run out of space.
That this over allocation is even possible, is because proxmox uses an lvm-thin pool for it's vm data [1], which means that blocks are only reserved when actually written to and since fallocate doesn't actually write anything, almost no storage is used up.

[0] https://man7.org/linux/man-pages/man1/fallocate.1.html
[1] https://pve.proxmox.com/wiki/Storage:_LVM_Thin

 

[manolis11]

Thank you Folke,

so from the moment that proxmox uses lvm-thin pool for vm data what is the purpose during the creation of the VM to set a value for disk space from the moment that the blocks are only reserved when actually written?
I can create all my VM with 1GB of hard disk and as im working with them slowly and add files inside the storage will be increased automatically .

 

[sb-jw]

I'm not sure if I understood you correctly, but the hard drive doesn't automatically increase in size with use. You create your virtual hard drives normally and give them a size. Only the actual usage within the virtual hard drive is counted towards the total storage.

 

[manolis11]

excuse me, the fact is that im confused because i have a physical 250GB SSD drive inside my server and for test purposes i have create 2 VM assigned them 300GB of disk size to each.
Now how can i know how many effective disk space i have left so i can use and from where those VM are taking this disk space?