I have a new HDD attached to my machine (CentOS 7.6), 1TB in size.
fdiskto create a new primary partition there of size 500GB, type "8e" (Linux LVM).
I then marked it as an LVM physical volume:
After that I created a volume group and added the partition to it:
vgcreate vg1 /dev/<partitionID>
On top of that I created a logical volume that used up all the available space of the volume-group:
lvcreate -n lv1 -l 100%FREE vg1
I then converted this logical volume to LUKS format:
cryptsetup luksFormat /dev/vg1/lv1
Finally on top of that I added an ext4 filesystem:
It worked pretty well and, using the passphrase set up during LUKS format and with some
/etc/crypttab magic, I was able to permanently mount it in
All is fine, except an interesting problem. The command
df -hT shows me something like this:
Filesystem Type Size Used Avail Use% Mounted on //.................. //.................. /dev/mapper/vg1-lv1 ext4 493G 73M 467G 1% /mnt/my_new_data
How come it says 493GB instead of 500GB? Also (most importantly): why is there only 467GB available? I didn't add any files yet in that directory. Since when is 467GB + 73MB = 493GB ?
Keep in mind that:
pvsreports 500GB as size for the physical volume
vgsreports 500GB as size for the volume group
lvsreports 500GB as szie for the logical volume
lsblkreports 500GB as size for that partition
Every tool says my disk should be 500GB in size. Except apparently I only have 467GB available.
What am I missing?
Could the LUKS encryption have anything to do with it?
EDIT: I may have found something. During creation of ext4 filesystem, there is something like this:
//...... Block size=4096 (log=2) //...... 32768000 inodes, 131070976 blocks 6553548 blocks (5.00%) reserved for the super user //......
Well, 6553548 x 4KB per block size ~= about 25GB. This might explain a big part of what happened, though the math still doesn't really add it.
Which now begs the question what is that "reserved for the super user" part? Why is this needed?