Drive Space

[Steve Laurie]

Hi,

Just wondering if someone could explain what's going on with my drive  
space.

I've got 2 x 500GB and 1 x 1.5TB SATA drives.

/dev/ad5p1 on /data/backup (ufs, NFS exported, local, soft-updates)
/dev/ad6s1a on /data/windows (ufs, NFS exported, local)
/dev/ad7s1a on /data/Public (ufs, NFS exported, local)

What I can't understand is why when I do 'df -h' or 'df -k' the Used  
and Avail amounts don't add up to the Size:
# df -h
Filesystem     Size    Used   Avail Capacity  Mounted on
/dev/ad5p1     1.3T    131G    1.2T    10%    /data/backup
/dev/ad6s1a    451G    113G    302G    27%    /data/windows
/dev/ad7s1a    451G     31G    384G     8%    /data/Public

# df -k
Filesystem  1024-blocks      Used      Avail Capacity  Mounted on
/dev/ad5p1   1442083968 137575328 1290087804    10%    /data/backup
/dev/ad6s1a   473012182 118932864  316238344    27%    /data/windows
/dev/ad7s1a   473012182  32967330  402203878     8%    /data/Public



Can someone explain where I'm going wrong?