50 GB missing on 931GB harddrive

with df -h

Filesystem      Size  Used Avail Use% Mounted on
/dev/sda3       911G   11G  854G   2% /home

with lsblk

sda           8:0    0 931,5G  0 disk 
├─sda1        8:1    0   5,9G  0 part 
└─sda3        8:3    0 925,7G  0 part /home

im a bit confused, please help

Solution 1:

When an ext4 filesystem is created, 5% of block is reserved by default. In your case, it accounts for 911*0.05=45.55.

You can modify the percentage number to, say 1%, as follows:

sudo tune2fs -m 1 /dev/sda3

The reserved blocks are used for this (from man tune2fs):

Set the percentage of the filesystem which may only be allocated by privileged processes. Reserving some number of filesystem blocks for use by privileged processes is done to avoid filesystem fragmentation, and to allow system daemons, such as syslogd(8), to continue to function correctly after non-privileged processes are prevented from writing to the filesystem. Normally, the default percentage of reserved blocks is 5%.