Why can fdisk create a 10GiB partition on a 10GiB device but not a 9.9GiB partition

I am currently learning about creating and managing devices on my Ubuntu Server 22.04 VM running on a KVM host. I added a virtual qcow2 storage disk to the VM. I used fdisk and for very specific purposes I need to create a 9.9GiB partition . I setup a DOS style partition table and then tried to create a primary partition.

Following are the steps to reproduce

root@hmxserver:/home/hmx# fdisk /dev/vdb

Welcome to fdisk (util-linux 2.37.2).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.


Command (m for help): o
Created a new DOS disklabel with disk identifier 0xb4942d4d.

Command (m for help): n
Partition type
   p   primary (0 primary, 0 extended, 4 free)
   e   extended (container for logical partitions)
Select (default p): 

Using default response p.
Partition number (1-4, default 1): 
First sector (2048-20971519, default 2048): 
Last sector, +/-sectors or +/-size{K,M,G,T,P} (2048-20971519, default 20971519): +9.9G
Value out of range.
Last sector, +/-sectors or +/-size{K,M,G,T,P} (2048-20971519, default 20971519): 

Created a new partition 1 of type 'Linux' and of size 10 GiB.

Questions That I want to ask?

1. Why is a 10GiB partition possible but 9.9GiB is out of range ?
2. I was able accomplish this on another VM with similar specs but on VirtualBox with a 10GiB VMDK disk attached and repeated the same steps. Why was fdisk successful there?
3. Does it have to do with the qcow2 devices ?
4. Or is it some sort of Floating Point Errors ?

I did a little experimentation, and to me it looks like fdisk cannot deal with floating point numbers, so +9.9G cannot be parsed while +9G or +10G can. That means that the error message was wrong.

If all you want is to use all the rest of the disk, simply do not enter a value for the partition end, and use the proposed default instead.

Otherwise, you might have to resort to a smaller unit and enter something like +10137M (i.e. 9.9*1024; remember this is using 1024-based units!).

Alternatively, you could use parted or its GUI version gparted.