Reserved space for root on a filesystem - new question

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http://unix.stackexchange.com – After googleing a little I find that ext4 reserve 5% for root Link - Link And if I use 'tune2fs' to set blocks to '0' will take them back. But I use that code on 100m volume and get half of the reserving space it was '10.5m' after set to '0' used space become '5.5m' Why used space still not zero? I used the following command to set the reserved space: tune2fs -m 0 /dev/mapper/truecrypt1 (HowTos)