redis_stats.rb

class RedisStats
  attr_reader :redis, :pattern

  def initialize(redis, pattern = '*')
    @redis = redis
    @pattern = pattern
  end

  def all_keys
    @all_keys ||= [].tap do |result|
      cursor = 0
      loop do
        cursor, keys = redis.scan(cursor, match: pattern)
        result.concat(keys)
        break if cursor == '0'
      end
    end
  end

  def debugs
    @debugs ||= debug_for(all_keys)
  end

  def debug_for(keys)
    keys.map do |key|
      debug = redis.debug(:object, key) rescue nil
      debug&.split(/:| /)&.drop(1)&.each_slice(2)&.to_h
    end.compact
  end

  def total_size
    @total_size ||= debugs.sum { |x| x['serializedlength'].to_i }
  end

  def avg_size
    total_size / debugs.size
  end

  def total_extrapolated_size
    all_keys.size * avg_size
  end

  def summary
    {
      keys_fetched: all_keys.size,
      keys_debugged: debugs.size,
      total_size: "#{(total_size.to_f / 1.gigabyte).round(2)} GB",
      avg_size: "#{(avg_size.to_f / 1.megabyte).round(2)} MB",
      total_extrapolated_size: "#{(total_extrapolated_size.to_f / 1.gigabyte).round(2)} GB",
    }
  end
end

# stats = RedisStats.new(Rails.cache.data, 'rails42:marketplace_offers:*')
# stats.summary