In some cases user is willing to sacrifice speed for space efficiency or better data security. I've developed driver for Directory that enables transparent compression (any transformation) of directory files, by using decorator pattern. With current experience, compression ratios are between 1:5 to 1:10, which depends on type of data stored in index.
Directory files are sliced into fixed chunks, each chunk separately transformed (eg. compressed, encrypted, ...) and written to supporting (nested) directory for storage.