path = Rails.root.join("tmp/cache/sprockets-lmdb-#{Rails.env}"); path.mkpath; env.cache = LMDBCacheStore.new(path.to_s)

lmdb_cache_store.rb

require 'lmdb'
require 'snappy'

class LMDBCacheStore
  attr_reader :max_size, :env, :db, :lru
  delegate :size, to: :db

  # The LMDB Gem has a bug where the Environment garbage collection handler will crash sometimes
  # if the environment wasn't closed explicitely before the reference was lost.
  # As a shitty workaround, we can make sure that we never lose a reference to the Environment by
  # putting it into a class variable.
  def self.save_env(env)
    @saved_envs ||= []
    @saved_envs << env
  end

  def initialize(path, max_size: 25_000)
    @max_size = max_size
    # mapsize is the absolute maximum size of the database before it will error out, it won't
    # allocate space on disk until it's needed.
    # According to the LMDB documentation, disabling sync means we lose the D in ACID (durability)
    # We don't care about that, because a missing transaction will
    # just lead to slightly more work being done the next time around.
    @env = LMDB.new(path, mapsize: 2.gigabyte, nometasync: true, nosync: true)
    self.class.save_env(@env)
    @db = env.database('cache', create: true)
    @lru = env.database('lru', create: true)
  end

  def get(key)
    value = nil
    env.transaction do
      value = db.get(key)
      update_lru(key) unless value.nil?
    end
    value && Marshal.load(Snappy.inflate(value))
  end

  def set(key, value)
    value = Snappy.deflate(Marshal.dump(value))
    env.transaction do
      db.put(key, value)
      update_lru(key)
      gc if lru.size > max_size * 1.5
    end
  end

  def inspect
    "#<#{self.class} size=#{size}/#{max_size}>"
  end

  def gc
    env.transaction do
      keys = []
      lru.each do |(hash, time)|
        keys << [time.unpack('q').first, hash]
      end
      return if keys.size <= max_size
      keys.sort!
      keys = keys.slice(0, keys.size - max_size)
      keys.each do |(_, key)|
        delete(key)
      end
    end
  end

  private

  def update_lru(key)
    lru.put(key, [Time.now.to_i].pack('q'))
  end

  def delete(key)
    db.delete(key)
  rescue LMDB::Error::NOTFOUND
    nil
  ensure
    lru.delete(key)
  end
end