5%的保留空间,在mkfs的时候可以改变比例。用-m选项,man里的说明
-m reserved-blocks-percentage
Specify the percentage of the filesystem blocks reserved for the super-user. This avoids fragmentation, and allows root-owned daemons, such as syslogd(8),to continue to function correctly after non-privileged processes are prevented from writing to the filesystem. The default percentage is 5%.