I'm using v19.07.6 on a WNDR3700v4.
I've used fdisk to create a single partition (as sda1 of type linux) on a 128GB usb stick and then used mkfs.ext4 to format as ext4. The stick is obviously now completely empty.
In LuCI, when I look under System / Mount Points:
under the 'Mount Points' heading, dev/sda1 shows as size 119.5GB. This is as expected.
under 'Mounted file systems', Available is 111.57 GB / 117.62 GB and Used = 0.05% (60.02 MB) .
I don't understand why 'Available' suggests approx 6GB is used when it isn't.
Earlier I had the Ksmbd package installed and I could write approx 117.5GB to sda1 (on a Windows 10 client) despite the above.
To resolve a separate issue I've earlier removed Ksmbd and installed the samba4-server package.
Now I can only write 111.5GB to sda1 suggesting Samba4 identifies the amount of space available in a different way to Ksmbd.
Is there anything obvious I'm doing wrong? It's almost like the Available space field has a bug in it where 117 is being displayed as 111.