Disk space gone

A

Abstract

Hi Everyone,

I bought two Western Digital Caviar Green Power hard disks of 2TB and 5400rpm.
I added those to the server through sysinstall. In fdisk I used the option A to
use the entire disk.

In the label editor I also assigned the maximum of diskspace.

When I look at the current disk space (df -h) of these hard disk it say 1.6TB avialable.
The size is 1.8TB.

I'm aware that claims the hard disk manafactures make aren't always right.
Due to the 1000byes in 1kb calculation or maybe other ways to calculate the space.

Having a 2TB disk and ending up with 1.6TB which is 400G less is a rather big difference.
Were did the 200G go which is between the size and the available space shown in the df command?
 
2TB = 2.000.000.000.000 => 1.8TB

Substract about 8% and you end up around 1.6TB.
 
I understand the first calculation but not the 8% which you remove from the 1.8TB to end up with 1.6TB.
It looks like a standard calculation, to get some knowledge about it where is this 8% used for?
 
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 performance:
  • Settings of 5% and less force space optimization to always be used which will greatly increase the overhead for file writes.
  • 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.
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