Dummy.file to me is an empty file with and allocated size on disk. With Exchange I create one on each LUN hosting mailbox databases or logs. Simple to create and just one line to your build document/script.
> fsutil.exe file createnew c:\dummy.file 20971520
Should a LUN fill up rapidly resulting in a dismounted database I can delete dummy.file and mount the database. I can then take action to reduce the database size or manually truncate logs. Coupled with Opalis you could delete dummy.file based on a SCOM disk alert, start online mailbox moves and send notifications without having to touch the server. Opalis could even raise a PO for more storage. :-)
Obviously LUNs should be sized correctly in the first place, but in large dynamic infrastructures this last resort can prove useful and give some breathing space.
You can also modify the Exchange database size limit using the registry, http://technet.microsoft.com/en-us/library/bb232092.aspx. this approach works, but isn’t a simply to restore service as deleting a file.
I hope you find this as useful as I have other the years.
Daniel Noakes