A guess...
information_schema
is contained in a single "tablespace" with shared "data_free". That is, it is as if you had innodb_file_per_table=OFF
, but only for those tables. A clue was "tables had the same 3783262208". This is what happens with OFF -- the free space is not for any individual table, but for all of them combined.
3GB out of 6TB is insignificant; don't worry about it. Don't bother rebooting.
On the other hand, I would worry about 6TB/8TB of disk being used. That could make it risky to ALTER
a large table if it has to "copy".
innodb_file_per_table
? Do all the tables claim to have 3.7GB of Data_free?