HDD Sizes reported wrong?

[Dan Nelson]

In the last episode (Jul 11), Steven Adams said:
> I have a LSI MegaRaid Raid card with 5x 36gig raid 5 scsi drives..
> 
> Now my /home is getting reported at 108gig(which is right) But for
> some reason its saying 97gig free and 2.4 gig used..
> 
> Im confussed where the other 8.5gig or so is gone??
> 
> /dev/amrd0s1h   108G   2.4G    97G     2%    /home

First, I recommend not using the -h option when trying to total things
up.  You lose lots of precision.  Second, the df values don't total up
because 8% is reserved on ffs filesystems so the disk allocation
algorithms stay efficient.  You can lower it with tunefs but as the
disk gets closer to 100%, performance rapidly degrades and files will
get fragmented.  Root can use that 8% which gives you the added benefit
that if a user fills up a drive it doesn't keep root processes from
working. 

http://www.freebsd.org/doc/en_US.ISO8859-1/books/faq/disks.html#DISK-MORE-THAN-FULL

-- 
	Dan Nelson
	dnelson at allantgroup.com