Don't understand df/du output

[Alain AUDEBERT aka 2A]

Hello list,

I having a problem to understand the output of du and df command :

[root at ftp ~]# df -h /opt/
Filesystem    Size    Used   Avail Capacity  Mounted on
/dev/xbd6     387G    342G     13G    96%    /opt

[root at ftp ~]# du -sh /opt/
342G    /opt/

But 387Go - 342Go not equal to 13Go ! Where's the available space ?
(same thing with df -k)

I have try a tunefs -m 1 /dev/xbd6, unmount, mount and nothing change
Is it the 8% reserved by FFS ?

Regards,

Alain

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