Use df on LUKS encrypted device

Question

I am using LUKS for disk encryption for a Ubuntu 18.04 system. I'd like to find out how much free space is still available on my hard drive. gparted prints the following:

As you can see the entire disk is subdivided into three partitions. On startup LUKS tells me that the encrypted disk is called nvme0n1p3_crypt. However, there are nvme0n1p1 and nvme0n1p1 as well.

1. Does this mean the are three partitions being all encrypted with the same key?

   The df -a -h command outputs among others the following:

   /dev/nvme0n1p2               705M  164M  490M  25% /boot
   /dev/nvme0n1p1               511M  6,1M  505M   2% /boot/efi
   

   Unfortunately, I cannot see the actual device which seems to be /dev/nvme0n1p3?

2. Why doesnt't df list the encrypted disk and how can I find out the available free space?

   df /dev/nvme0n1p3 -h yields the following:

   Filesystem      Size  Used Avail Use% Mounted on
   udev            7,7G     0  7,7G   0% /dev
   

3. This output seems weird, because the total available space is only 7,7G. This is not true as you can see above in the gparted output. The partition has roughly 475.75 GiB. Can you explain this?

Answer 1

Does this mean the are three partitions being all encrypted with the same key?

No, this means partition is in use - aka mounted. It's not recommended to edit mounted/in-use filesystems, since data loss can occur. The third big partition is not mounted according to the column in the screenshot

Why doesnt't df list the encrypted disk and how can I find out the available free space?

df reports usage for only mounted filesystems. Filesystems that are not mounted will report usage for /dev filesystem. Here's example of mounted vs unmounted reports:

# unmount my second drive, reports usage for udev
$ udisksctl unmount -b /dev/sdb1
Unmounted /dev/sdb1.
$ df  /dev/sdb1
Filesystem     1K-blocks  Used Available Use% Mounted on
udev             4000948     0   4000948   0% /dev
# statvfs call can be done on mounted filesystem, hence we see usage report
$ udisksctl mount -b /dev/sdb1
Mounted /dev/sdb1 at /mnt/ubuntu.
$ df  /dev/sdb1
Filesystem     1K-blocks      Used Available Use% Mounted on
/dev/sdb1      115247656 100662532   8707776  93% /mnt/ubuntu

The /dev/nvme0n1p3 does not seem to have proper mountpoint (directory) and instead reports ubuntu-vg. Try the following ( source )

$ sudo mount /dev/ubuntu-vg/root /mnt
$ df -h /dev/ubuntu-vg/root

Since you're using lvm you should use sudo lvs preferably to find out what logical volumes are actually called.

This output seems weird, because the total available space is only 7,7G. This is not true as you can see above in the gparted output. The partition has roughly 475.75 GiB. Can you explain this?

It can occur when filesystem is not resized after partitioning, even with non-lvm cases. So if you have recently resized/repartitioned that device it could be the cause. Again, probably you don't wanna use df with lvm.

Answer 2

IIRC GParted doesn't really understand LVM volumes. Try the Disks tool.

1. What does df / report? That would show the device mounted on /, which should be the encrypted LVM volume.
2. df /some/file checks the filesystem on which that file exists. So df /dev/nvme0n1p3 is just checking the filesystem mounted on /dev.
3. The filesystem on /dev is the udev virtual filesystem. Its size is the memory (RAM) you have (which would be around 8GB). That size has nothing to do with the hard disk size.