Where did 47gigabytes of space go?

Question

Clean installation od Debian squeeze 6.0.6 x64 on intel atom 230, with 3 HDDs, with 3x500GB.

df -h shows :

datastore              size used  free   %   mounted
/dev/md0              917G  909M  870G   1% /
tmpfs                1001M     0 1001M   0% /lib/init/rw
udev                  996M  188K  996M   1% /dev
tmpfs                1001M     0 1001M   0% /dev/shm

df -i shows :

datastore              size used  free   %   mounted
/dev/md0             61054976   27582 61027394    1% /
tmpfs                 256087       7  256080    1% /lib/init/rw
udev                  254831     692  254139    1% /dev
tmpfs                 256087       1  256086    1% /dev/shm

It is ext3.

Please tell me, where did my 47 gigabytes go? It is a clean installation as you can see.

du -sh shows: 709M

Answer 1

Does this help?

man mkfs.ext3
-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%.

Answer 2

47G from 917G is close to 5%. This is the default amount of reserved blocks used for super-user aims. You can see that executing tune2fs -l /dev/md0 | grep "Reserved block count". -m parameter of tune2fs and mke2fs can change the default behavior.