At 9:52 AM -1000 4/1/02, Clifton Royston wrote: >Some knowledgeable admins have suggested that the flag to disable >qpopper writing UIDLs back to the file is a performance benefit, >because it reduces disk I/O (at the cost of CPU) and most modern >systems are I/O bound not CPU bound. I haven't tried this out, because >when I make this change I think it means everyone who leaves mail on >our server would get it all downloaded again. (Then again, maybe >that's a *good* thing. Heh.)
When UIDs are written into the mail spool they are calculated using a hash of the message headers and a random component, to ensure uniqueness. When UIDs aren't written into the spool they need to be generated the same each time, so there can't be a random element. Thus, after making the switch pre-existing messages in the spool will have UIDs that contain a random element, while new messages won't. So there shouldn't be a problem with downloading old mail again. However, there is a greater chance of duplicate UIDs, which means that potentially some messages might not get downloaded (since they seem to be the same as an earlier message to the client).
