The database server recently ran out of space and caused a huge amount of DB corruption.
I was in the forunate position of having a clone of the database server that was a week old so was able to get the sites up and running (albeit out of date quite quickly). I then took the most recent mysql backups from the corrupt server and started applying them to the clone (in order to keep all the sites up and running during the recovery time).
This has all gone well in that the databases are coming back with no real problems (bar the occasional massive growth of the mdl_backup_controllers table which I then need to truncate and optimize).
But the moodle_data directories on the application server are growing quite dramatically during the restore process (10's of GB's over the multiple sites) - could anyone shed any light on why this would happen and if there's anyway for me to reclaim the space that's been lost?