When I run df -B G I see the following line in the result:

Filesystem  1G-blocks  Used   Available
/dev/sda    915G        3G     867G

First of all my hard disk drive is 1T so why 915G of it is usable? second of all if 3G is used why 867G is available instead of 912G? (It's a new Debian installation and I formatted the hard drive during the installation)

  • 9
    The disk maker sells you 1000*1000*1000*1000 bytes as 1T. df reports this as 915*1024*1024*1024. Furthermore, the filesystem seems to have 45GiB overhead. Jun 30, 2021 at 22:14
  • 3
    The "overhead" is the installation of Debian. Jun 30, 2021 at 22:20
  • 3
    ext4 has 5% reserved by default this accounts for the bulk of the overhead.
    – A.B
    Jun 30, 2021 at 22:32
  • 2
    Yes. Between the file system reservation and the OS, all of the space is accounted for. Jun 30, 2021 at 22:37
  • 3
    Additional usage comes from copies of the superblock, group descriptors, inodes, etc. Read man mke2fs.
    – waltinator
    Jun 30, 2021 at 23:23


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