[CentOS] MySQL/file system question

Hello listmates,

I got a rather strange situation that I can't quite make sense of. OK, I've
got a very large data file to sort (hundreds of millions of lines) and I
decided to use MySQL for the purpose. I inserted the stuff into a table
easily enough. Then I decided to sort it and got stuck as it turned out
that MySQL, unless specifically configured to do otherwise, puts temporary
files in /tmp which simply was not sufficiently large. Then I changed that
directory to a partition that had more space (let's call the new temp
driectory /home/big-temp) and now as I am running the query aimed at
sorting the data it seems like space, according to the df, is no longer
being used up under / (which was there /tmp was) but is now being used up
in the right partition ( /home, the large one). Yet /home/big-temp is still
empty!

So how is that possible? I know there are all kinds of file locking
possible depending on the circumstances, file system type, etc - but how
can it be that the files, if they exist, are not even visible to ls, even
"ls -a", even run by root?

Thanks in advance for any clarification.

Cheers,

Boris.