Consumed feeds (simplepie)..

type of data: XML (RSS)
data structure: simplepie internal (files in json format)
when it gets stored: On field retrieval (optimised for retrival in cron)
where it gets stored: moodledata/cache/simplepie/
how it gets read: simplepie internally
does it need locking: no
how it gets cleared: The time to live expires (forced by cron) aka never.
typical sizes: 5K per feed?
safeguards in place (eg limits):
Notes: Can definitely be shared between requests, in fact its designed so that we never retrieve feeds in an interactive request

Generated feeds (forum)..

type of data: XML (RSS)
data structure: generated rss
when it gets stored: When generating rss feed
where it gets stored: moodledata/cache/rss/
how it gets read: When serving feeds
does it need locking: no/yes(?!)
how it gets cleared: It doesn't, gets regenerated when expires
typical sizes: varies (5k?)
safeguards in place (eg limits):