does not understand "df -H"

[Sahil Tandon]

Tsu-Fan Cheng <tfcheng at gmail.com> wrote:

> Hi,
>    I have a pretty basic question: when I "df -H" my disk, the numbers
> cant add up,
> 
> 
> Filesystem     Size    Used   Avail Capacity  Mounted on
> /dev/ad0c      484G    429G     17G    96%    /ad0c
> 
> 
> there should be more disk space available based on what is shown. why
> is this like that??

This is default behavior.  From tunefs(8):

-m minfree
	     Specify the percentage of space held back from normal users; the
	     minimum free space threshold.  The default value used is 8%.
	     Note that lowering the threshold can adversely affect perfor-
	     mance:

	     +o	 Settings of 5% and less force space optimization to always be
		 used which will greatly increase the overhead for file
		 writes.

	     +o	 The file system's ability to avoid fragmentation will be
		 reduced when the total free space, including the reserve,
		 drops below 15%.  As free space approaches zero, throughput
		 can degrade by up to a factor of three over the performance
		 obtained at a 10% threshold.

Your df output suggests your minfree is set to the default 8%; to
confirm this:

% dumpfs /dev/ad0c | grep minfree | cut -f 1-2

-- 
Sahil Tandon <sahil at tandon.net>