// The names are repetitive so re-use the same builder to avoid allocations.
      val fileBuilder = StringBuilder(key).append('.')
      val truncateTo = fileBuilder.length
      for (i in 0 until valueCount) {
        fileBuilder.append(i)
        cleanFiles += directory / fileBuilder.toString()
        fileBuilder.append(".tmp")
        dirtyFiles += directory / fileBuilder.toString()
        fileBuilder.setLength(truncateTo)
      }