How to: Make a fast lookup able storage directory structure

Partitioning

If you have many files dumped in the same directory disk lookups might be slow.
That is why it's a good idea to partition your files into directories like in the following example:

class FileUploader < CarrierWave::Uploader::Base
  storage :file
  
  def store_dir
    "documents/#{model.file_name[0, 2]}/#{model.id}"
  end  
end

In this example we use the first two letters of the filename to create directories that will partition our files.
This way we can have nice fast dictionary-like lookups.

See also: http://stackoverflow.com/questions/446358/storing-a-large-number-of-images

Cleanup

It is also a good idea to clean up your empty directories when the files in them are deleted, like in the following example:

class FileUploader < CarrierWave::Uploader::Base
  storage :file
  after :remove, :delete_empty_upstream_dirs

  def store_dir
    "#{base_store_dir}/#{model.id}"
  end
  
  def base_store_dir
    "documents/#{model.file_name[0, 2]}"
  end

  def delete_empty_upstream_dirs
    path = ::File.expand_path(store_dir, root)
    Dir.delete(path) # fails if path not empty dir
    
    path = ::File.expand_path(base_store_dir, root)
    Dir.delete(path) # fails if path not empty dir
  rescue SystemCallError
    true # nothing, the dir is not empty
  end
end