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bitcoin/doc/assumeutxo.md

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Assumeutxo Usage

Assumeutxo is a feature that allows fast bootstrapping of a validating bitcoind instance.

For notes on the design of Assumeutxo, please refer to the design doc.

Loading a snapshot

There is currently no canonical source for snapshots, but any downloaded snapshot will be checked against a hash that's been hardcoded in source code. If there is no source for the snapshot you need, you can generate it yourself using dumptxoutset on another node that is already synced (see Generating a snapshot).

Once you've obtained the snapshot, you can use the RPC command loadtxoutset to load it.

$ bitcoin-cli -rpcclienttimeout=0 loadtxoutset /path/to/input

After the snapshot has loaded, the syncing process of both the snapshot chain and the background IBD chain can be monitored with the getchainstates RPC.

Pruning

A pruned node can load a snapshot. To save space, it's possible to delete the snapshot file as soon as loadtxoutset finishes.

The minimum -prune setting is 550 MiB, but this functionality ignores that minimum and uses at least 1100 MiB.

As the background sync continues there will be temporarily two chainstate directories, each multiple gigabytes in size (likely growing larger than the downloaded snapshot).

Indexes

Indexes work but don't take advantage of this feature. They always start building from the genesis block and can only apply blocks in order. Once the background validation reaches the snapshot block, indexes will continue to build all the way to the tip.

For indexes that support pruning, note that these indexes only allow blocks that were already indexed to be pruned. Blocks that are not indexed yet will also not be pruned.

This means that, if the snapshot is old, then a lot of blocks after the snapshot block will need to be downloaded, and these blocks can't be pruned until they are indexed, so they could consume a lot of disk space until indexing catches up to the snapshot block.

Generating a snapshot

The RPC command dumptxoutset can be used to generate a snapshot for the current tip (using type "latest") or a recent height (using type "rollback"). A generated snapshot from one node can then be loaded on any other node. However, keep in mind that the snapshot hash needs to be listed in the chainparams to make it usable. If there is no snapshot hash for the height you have chosen already, you will need to change the code there and re-compile.

Using the type parameter "rollback", dumptxoutset can also be used to verify the hardcoded snapshot hash in the source code by regenerating the snapshot and comparing the hash.

Example usage:

$ bitcoin-cli -rpcclienttimeout=0 dumptxoutset /path/to/output rollback

For most of the duration of dumptxoutset running the node is in a temporary state that does not actually reflect reality, i.e. blocks are marked invalid although we know they are not invalid. Because of this it is discouraged to interact with the node in any other way during this time to avoid inconsistent results and race conditions, particularly RPCs that interact with blockstorage. This inconsistent state is also why network activity is temporarily disabled, causing us to disconnect from all peers.

dumptxoutset takes some time to complete, independent of hardware and what parameter is chosen. Because of that it is recommended to increase the RPC client timeout value (use -rpcclienttimeout=0 for no timeout).