+
+ def index
+ # Retrieve cache age histogram info from logs.
+
+ # In the logs we expect to find it in an ordered list with entries
+ # of the form (mtime, disk proportion free).
+
+ # An entry of the form (1388747781, 0.52) means that if we deleted
+ # the oldest non-presisted blocks until we had 52% of the disk
+ # free, then all blocks with an mtime greater than 1388747781
+ # would be preserved.
+
+ # The chart we want to produce, will tell us how much of the disk
+ # will be free if we use a cache age of x days. Therefore we will
+ # produce output specifying the age, cache and persisted. age is
+ # specified in milliseconds. cache is the size of the cache if we
+ # delete all blocks older than age. persistent is the size of the
+ # persisted blocks. It is constant regardless of age, but it lets
+ # us show a stacked graph.
+
+ # Finally each entry in cache_age_histogram is a dictionary,
+ # because that's what our charting package wats.
+
+ @cache_age_histogram = []
+ @histogram_pretty_date = nil
+ histogram_log = Log.
+ filter([[:event_type, '=', 'block-age-free-space-histogram']]).
+ order(:created_at => :desc).
+ limit(1)
+ histogram_log.each do |log_entry|
+ # We expect this block to only execute at most once since we
+ # specified limit(1)
+ @cache_age_histogram = log_entry['properties'][:histogram]
+ # Javascript wants dates in milliseconds.
+ histogram_date_ms = log_entry['event_at'].to_i * 1000
+ @histogram_pretty_date = log_entry['event_at'].strftime('%b %-d, %Y')
+
+ total_free_cache = @cache_age_histogram[-1][1]
+ persisted_storage = 1 - total_free_cache
+ @cache_age_histogram.map! { |x| {:age => histogram_date_ms - x[0]*1000,
+ :cache => total_free_cache - x[1],
+ :persisted => persisted_storage} }
+ end
+
+ # Do the regular control work needed.
+ super
+ end