Thumbnail caching proposal

[Akara Sucharitakul]

As we run into filesystem bottlenecks, it makes sense to reduce the 
filesystem access by caching thumbnail images in memcached. The 
thumbnails are about 6-7 KB in size. We want to ensure there's a hot set 
of thumbnails being cached in memcached but not that every image gets 
cached. So here's my first draft on how to achieve that:

1. All thumbnails that are accessed by the client and driver are cached 
in memcached with a timeout of 90 seconds. Of course, we can still 
adjust this. Any access to the thumbnails will check the cache at first.

2. In order to keep the hot set low, we will modify our tag search query 
to order by event date with the most recent upcoming events first. As a 
result, the set of events searched by each tag are more deterministic 
and will not often change. Since the tag search only shows 10 events per 
page, the thumbnails for these top ten events for each tag will be held 
in memcached.

The mathematical model for this is a little complicated, though. Since 
we're dealing with non-uniform distribution of tags and a cap of 10 
upcoming events on top of that. But please let me know whether you agree 
with this conceptually.

Thanks,
-Akara