2.6.1.3.2 Single-Instance Storage

As seen in section 2.6.1.2.4, the NDB Layer supports single-instance storage by having reference counts associated with each data block. Additional references to the same BID can be held as multiple nodes hold references to the same BID. This, combined with the immutability of the NDB store, allows new versions of a particular modified copy to be persisted in a new BID while all the other un-modified copies continue to refer to the old data.