Qu Wenruo wrote:

If you're really interesting in whether your fs has skinny metadata 
enabled, you can check btrfs-show-super output.

Like the following output indicates skinny metadata:
------
incompat_flags		0x161
			( MIXED_BACKREF |
			  BIG_METADATA |
			  EXTENDED_IREF |
			  SKINNY_METADATA ) <<<Here
------

Even it has skinny metadata, it's still possible that some metadata
are still in old format if you used btrfstune to convert an old fs to
skinny metadata.

It was a freshly created filesystem. However, btrfs-show-super shows it
does *not* have skinny metadata:

incompat_flags		0x61
			( MIXED_BACKREF |
			  BIG_METADATA |
			  EXTENDED_IREF )

Maybe gparted explicitely requested it to be created without skinny
metadata. That won't make me lose my sleep, though.

But anyway, it's always good to see the problem solved.

Indeed :-)

Thanks again