ZFS uses half of the RAM size as file cache per default. ZFS writes to disk only every 5 seconds (write flush). Therefore the RAM is heavily used as cache.
Got it thanks!
BTW: during copying and maybe 30 minutes after - 55-60% RAM was used, but after that - again my previous result 20% used.
Not sure if I add one more 6TB drive to stripe should I add extra memory? Now it seems that 1TB=1GB of RAM, probably 8GB not enough for 9TB stripe from 2 disks.
I would say that this is not ZFS related and must have other reasons. If one file is copied from ext4 to ZFS you must see one additional file on ZFS.
It`s very strange, because I have less folders and files on ZFS and I have tried copying a few big folders with many files/folders inside from one ext4 drive to another ext4 drive - size and amount of folders/files were equals (==).
And all these experiments I done 10 hours ago, few minutes ago tried the same - check amount files/folders on source (ext4) and destination (zfs) - and they are equal, probably some data was cached in RAM.
But anyway I can`t see 'ZFS you must see one additional file on ZFS'
Very interesting behavior.