How does FreeBSD calculate disk sizes

Alex de Kruijff freebsd at
Mon Nov 3 16:39:00 PST 2003

On Mon, Nov 03, 2003 at 12:07:15PM +0300, Odhiambo Washington wrote:
> Hello users,
> I have a disk which is actually 72GB. 2GB has been used as swap while
> the rest was given to /.
> Can someone explain to me what I could be missing here, because what
> I am seeing isn't what I expect. Perhaps it's just right while I am
> the dumb one. Why isn't the whole size reported?
> sucks# uname -nmr
> 5.1-RELEASE-p10 i386
> sucks# df -h
> Filesystem    Size   Used  Avail Capacity  Mounted on
> /dev/da0s1a    64G   1.8G    57G     3%    /
> devfs         1.0K   1.0K     0B   100%    /dev
> Some expert explanation would help clear my ignorance!

The answers can be found in the FAQ. The source is:

9.25. How is it possible for a partition to be more than 100% full?

A portion of each UFS partition (8%, by default) is reserved for use by
the operating system and the root user. df(1) does not count that space
when calculating the Capacity column, so it can exceed 100%. Also,
you'll notice that the Blocks column is always greater than the sum of
the Used and Avail columns, usually by a factor of 8%.

For more details, look up the -m option in tunefs(8).


Articles based on solutions that I use:

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