The following question was asked on the internal alias:
Q: When I open a workbook in the EWA web part via URL (say, to a \\unc\file\location), does that file ever get stored in the WSS DB in some interim step, or does it just open directly in memory?
The real reason I’m asking is that I’m curious if workbooks opened this way would be subject to WSS file storage limits.
A: The answer to both questions is no. :)
No: We do not save the file in WSS.
No: We do not load it directly streamed from memory.
What we do is bring the file first into the local disk into a cache location that's set up when the server is installed. That location is maintained throughout the life of the server, including files being removed when the server boots up/shuts down and general maintenance that occurs during the service run-time.