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Is it possible that over weeks, it's still keeping in memory and not writing?
Probably not.
If the database has nothing better to do, then it
will tidy up after itself and push things out to disk. The important thing, though, is that it's not
guaranteed to do so.
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is it possible that it's doing something to the data even when no one is accessing it ...
... like analysing it, collecting statistics on it and probably lots of other things that databases do for their own benefit and/or amusement. :-)
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mysqldump ... been doing that every 6 months as another form of backup
No; you've actually been
taking Backups that you stand a chance of Recovering your database with (which, after all, is the whole, and
only, point of taking backups!). Nothing else you've been doing (with disk files) really counts.
Regards, Phill W.