High level overview of UTxO-HD

UTxO-HD is an internal rework of the Consensus layer that features a hybrid database for Ledger State data. UTxOs are stored in a separate database that can be backed by an on-disk database or with an in-memory implementation.

Each of those backends have specific behaviors and implications, so we will refer to them individually by InMemory and OnDisk.

End-users of the InMemory backend (the default one) should not appreciate any major difference in behavior and performance with respects to a pre-UTxO-HD node.

End-users of the OnDisk backend will observe a regression in performance. For now the OnDisk backend is implemented via LMDB and not optimal in terms of performance, but we plan on making use of the LSM trees library that Well-Typed is developing for a much better performance. In particular operations that need UTxOs (applying blocks/transactions) will have the overhead of a trip to the disk storage plus some calculations to bring the disk values up to date to the tip of the chain.

In exchange for that performance regression, a Cardano node using the OnDisk backend can run with much more modest memory requirements than a pre-UTxO-HD node.

In terms of functionality, both backends are fully functional.

For a more extensive description of UTxO-HD, see the full documentation.