ZFS inodes issue (0 reported by df -hi)

[Miroslav Lachman]

I am stresstesting ZFS filesystem on my test machine and sometimes see 
wrong output of df in inodes columns - reporting zero used inodes when 
more than 53 milions are used.

Bellow is df -hi reports taken in few seconds / minutes (there are 
running some heavy copying tasks in the background)
[other partitions was stripped for clarity]

root at cage ~/# df -hi
Filesystem  Size    Used   Avail Capacity iused  ifree %iused  Mounted
tank        364G    362G    1.9G    99%       0  15915    0%   /tank
                                           ^^^^^^        ^^^^^^
root at cage ~/# df -hi
Filesystem  Size    Used   Avail Capacity  iused  ifree %iused  Mounted
tank        364G    362G    1.9G    99% 53382659  15757  100%   /tank

root at cage ~/# df -hi
Filesystem  Size    Used   Avail Capacity  iused  ifree %iused  Mounted
tank        364G    362G    1.9G    99% 53391685  15503  100%   /tank

root at cage ~/# df -hi
Filesystem  Size    Used   Avail Capacity iused  ifree %iused  Mounted
tank        364G    363G    1.3G   100%       0  10965    0%   /tank
                                           ^^^^^^        ^^^^^^
root at cage ~/# df -hi
Filesystem  Size    Used   Avail Capacity  iused  ifree %iused  Mounted
tank        364G    363G    1.3G   100% 53591981  10817  100%   /tank

root at cage ~/# df -hi
Filesystem  Size    Used   Avail Capacity iused  ifree %iused  Mounted
tank        364G    363G    1.0G   100%       0   8267    0%   /tank
                                           ^^^^^^        ^^^^^^
root at cage ~/# df -hi
Filesystem  Size    Used   Avail Capacity  iused  ifree %iused  Mounted
tank        364G    363G    1.0G   100% 53672433   8245  100%   /tank


Next thing that I do not understand is how ZFS uses inodes? The total 
number of inodes (iused+ifree) grows by the time as filesystem is more 
and more filled.

Version: FreeBSD 7.0-RELEASE-p2 #0: Wed Jun 18 06:48:16 UTC 2008 
root at amd64-builder.daemonology.net:/usr/obj/usr/src/sys/GENERIC  amd64

Miroslav Lachman