Deduplication can be performed in 4 ways:
1. Immediately when the write is processed (inline) and before the write is ackowledged back to the originator of the write (pre-ack).
2. Immediately when the write is processed (inline) and in parallel to the write being acknowledged back to the originator of the write (on-ack).
3. A short time after the write is processed (inline) so after the write is acknowleged back to the originator of the write - eg. when flushing the write buffer to persistent storage (post-ack)
3. After the write has been committed to the persistent storage layer (post-process).
The first and second methods, when properly integrated into the solution, are most likely to offer both performance and capacity benefits. The third and fourth methods are primarily used for capacity benefits only.
Windows Server 2019 deduplication is performed outside of IO path (post-processing) and is multi-threaded to speed up processing and keep performance impact minimal.