BTRFS: more space available than free

[Klaus Ethgen]

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Hello,

currently, I suffer of a weird problem with a btrfs filesystem:
   ~> uname -r
   4.0.9
   ~> stat -f /
     File: "/"
       ID: a446833a80ceeb6a Namelen: 255     Type: btrfs
   Block size: 4096       Fundamental block size: 4096
   Blocks: Total: 128472     Free: 41964      Available: 42981
   Inodes: Total: 0          Free: 0

As you can see, f_bavail is slightly bigger than f_bfree. As I
understand the values, f_bavail should be never bigger than f_bfree.

Currently that breaks check_disk from nagios.

How can that happening? I found a thread about some try to make f_bavail
more accurate in 2014 but nothing about f_bfree.

What I didn't try until now, is, if that problem also happened with the
most recent kernel version but this will be my next step.

Please forgive that I don't read lkml regularly. Please keep me in Cc
for this reason.

Regards
   Klaus
- -- 
Klaus Ethgen                              http://www.ethgen.ch/
pub  4096R/4E20AF1C 2011-05-16   Klaus Ethgen <Klaus@xxxxxxxxx>
Fingerprint: 85D4 CA42 952C 949B 1753  62B3 79D0 B06F 4E20 AF1C
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