Hi, i’ ve just created a mirror with my new 2x 8To HDD. (7,2 To usable in theory)
(my machine is a HP G8 microserver with Truenas 25.10.5 - Goldeye, last update at this time)
On my shared SMB i’ve mesured 20,7 To of size of data used with my favorite app totalcommander on windows. Windows explorer report 18,8 To of data.
The pool has no snapshot, compression is LZ4 and record size is 128 KiB.
The size reported is strange, see picture attached below, anyone has a idea on this result ?
I am guessing you are coming across block cloning or Deduplication.
Try running the following and posting the results back in a Preformatted Text mode. Icon looks like </> on reply toolbar.
zpool list -o name,size,cap,alloc,free,bcloneratio,bcloneused,bclonesaved [POOL_NAME]
Hi SmallBarky, result of your command below :
NAME SIZE CAP ALLOC FREE BCLONE_RATIO BCLONE_USED BCLONE_SAVED
TEST 7.27T 17% 1.28T 5.99T 14.47x 1.26T 17.0T
It’s block cloning. Try searching the internet for something like ‘zfs block clone’ for an explanation.
I don’t have any links to post to explain it. It has been discussed in the past on the forum. It makes for very strange space reporting but I’m not sure if it’s an item that can be improved at this time. The command line is the best way to get the most accurate view of space.
Thank you very much for your help SmalBarky 
In short you managed to store 17 TB of (highly redundant) data on your (newly created, really?) 8 TB pool before noticing that there was something strange. 
But ZFS magic has its limits: At 75% full it is due time to plan for increasing this storage.
yes lol
But this machine and this pool named “TEST” are just here for test purpose.
it is filled with many doubloned ISO image files.