The reserved space is a portion of the filesystem that is only accessible to the root user or users with sudo privileges. The reserved space serves two purposes: to prevent ordinary users from filling up the filesystem completely and crashing the system, and to reduce fragmentation by leaving some free space. The reserved space is set by default to 5% of the total filesystem size when the filesystem is created with the mkfs.ext4 command. This means that 5% of the blocks are reserved for the root user and are not counted as available space by the df command. The reserved space can be changed or disabled by using the tune2fs command with the -m option. For example, to change the reserved space to 1%, use the following command:

sudo tune2fs -m 1 /dev/sdXY

where /dev/sdXY is the device name of the ext4 filesystem.

To disable the reserved space completely, use 0% instead of 1%. However, this is not recommended, as it may affect the performance and stability of the system, especially if the filesystem is almost full.