Requests for comment/Simplify thumbnail cache


 * TODO
 * Add more data/options from meeting notes
 * Send link to Aaron & Faidon
 * Move to RFC namespace proper
 * Announce on wikitech-l, etc

Something about thumbnail delivery pipeline


 * streamline varnish cache purge process
 * reduce disk storage footprint of thumbnails
 * remove need to enumerate all generated thumbs

Things we are not happy about:


 * thumbs take up 60% (triple check number with Faidon) of the on disk storage footprint in swift
 * swift has been configured somewhat awkwardly to support wildcard listing of stored thumbs
 * php layer has extra complexity to hash thumb path into right swift collection (proper term?)
 * lots and lots of varnish purge messages may be needed to clean up the thumbs for a given media delete

could also discuss hashed image urls and versioned image urls as other aspects/solutions for the same problem.

Prop #1
Treat thumbs as a CDN only concern.
 * 1) stop storing in swift at all
 * 2) configure varnish so we can purge all at once

pros:
 * lots of disk reclaimed from varnish
 * no need to delete files from varnish
 * only one htcp purge message needed

cons:
 * increased utilization of image scalers (faidon has some reasonable sounding numbers)
 * increased latency for CDN misses