[plug] Dude, where is my HD space gone?

[Cameron Patrick]

Chris Caston <caston at arach.net.au> writes:

>> Will take the 5% root allocation explanation. I now remember reading about 
>
> Yeah I have to be really careful when selling harddrives. I tell the
> customer that they won't get the full 120 gig for various technical
> reasons.

There's several things going on there, beyond the 5% reserved blocks
mentioned above.

* Hard disc manufacturer gigabytes are 10^9 bytes (1000000000 bytes)
  whereas operating system gigabytes are 2^30 bytes (1073741824 bytes)
  so you really get 0.93 GiB per GB.  (Using the pedantically accurate
  SI prefixes which no-one actually uses: GiB "gibibytes" for 2^30 and
  GB "gigabytes" for 10^9.  The people who use these prefixes
  consistently probably also pronounce "giga" as "jai:ger" and believe
  that they'll see AI smarter than humans in their lifetime.)

* Some overhead is taken up by the filesytem for keeping track of
  which files are stored where on the disc and other "book-keeping"
  data.  This is why a filesytem has slightly fewer available blocks
  than the partition that it lives on (looks like approx 200mb on an
  11gb ext3 filesystem on my machine).

* Some filesystems (including ext2/3) also reserve some amount of space
  for root's use only, as discussed in this thread.

* As well as having a limited amount of space, most filesystems also
  have a finite number of inodes (files) that they can store too.
  Reiserfs is an exception here.  You can see this with df -i:

  Filesystem            Inodes   IUsed   IFree IUse% Mounted on
  /dev/hda6            4294967295       0 4294967295    0%

Cheers,

Cameron.