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After executing:
sudo mount -o subvolid=0 /dev/sdc3 /mnt/temp
mount shows this:
/dev/sdc3 on /mnt/temp type btrfs (rw,relatime,ssd,space_cache,subvolid=5,subvol=/)
Why subvolid is 5 despite I specified 0 ? I don't have any subvolume with this ID. What does it mean?