The partitions that contains the ext3 and ext4 filesystems reserve the 5% of the total size of the filesystm by default. The idea here is even when you run out of disk space, the root user should still be able to log in and system services should still run. Without this option, the root user could be not able to acces and “clean up” since the system may become unstable, trying to log to in a filesytem full at 100%, for example. The other reason is to help the general optimization with less fragmentation of the filesystem.
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