In addition to the memory requirements, I seem to recall that it works at the block level, as opposed to the file level. So you could have two of the same file,

In addition to the memory requirements, I seem to recall that it works at the block level, as opposed to the file level. So you could have two of the same file, but maybe one copy is written at the start of a block and one is written in the middle of a block. Same file, different blocks, so no deduplication.