We have to save the sessions index to disk from the DiskPageStore when we get a shutdown/destroy from the servlet container
(I hope that that that event will be called on a servlet container shutdown and on a undeploy/redeploy of the webapp.)
Then with a destroy we save it. When we are fired up again we read that file and also delete the file immediantly.
Then when the page store is created and the file isn't there it can delete all the page stores files that are currently on disk
because the destroy didn't happen so it was a crash or a hard kill and there won't be any sessions anyway.