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Add PoV-reclaim enablement guide to polkadot-sdk-docs (paritytech#4244)
This adds a small guide on how to prepare your parachain for pov-reclaim usage.
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| //! This guide will teach you how to enable storage weight reclaiming for a parachain. The | ||
| //! explanations in this guide assume a project structure similar to the one detailed in | ||
| //! the [substrate documentation](crate::polkadot_sdk::substrate#anatomy-of-a-binary-crate). Full | ||
| //! technical details are available in the original [pull request](https://github.com/paritytech/polkadot-sdk/pull/3002). | ||
| //! | ||
| //! # What is PoV reclaim? | ||
| //! When a parachain submits a block to a relay chain like Polkadot or Kusama, it sends the block | ||
| //! itself and a storage proof. Together they form the Proof-of-Validity (PoV). The PoV allows the | ||
| //! relay chain to validate the parachain block by re-executing it. Relay chain | ||
| //! validators distribute this PoV among themselves over the network. This distribution is costly | ||
| //! and limits the size of the storage proof. The storage weight dimension of FRAME weights reflects | ||
| //! this cost and limits the size of the storage proof. However, the storage weight determined | ||
| //! during [benchmarking](crate::reference_docs::frame_benchmarking_weight) represents the worst | ||
| //! case. In reality, runtime operations often consume less space in the storage proof. PoV reclaim | ||
| //! offers a mechanism to reclaim the difference between the benchmarked worst-case and the real | ||
| //! proof-size consumption. | ||
| //! | ||
| //! | ||
| //! # How to enable PoV reclaim | ||
| //! ## 1. Add the host function to your node | ||
| //! | ||
| //! To reclaim excess storage weight, a parachain runtime needs the | ||
| //! ability to fetch the size of the storage proof from the node. The reclaim | ||
| //! mechanism uses the | ||
| //! [`storage_proof_size`](cumulus_primitives_proof_size_hostfunction::storage_proof_size) | ||
| //! host function for this purpose. For convenience, cumulus provides | ||
| //! [`ParachainHostFunctions`](cumulus_client_service::ParachainHostFunctions), a set of | ||
| //! host functions typically used by cumulus-based parachains. In the binary crate of your | ||
| //! parachain, find the instantiation of the [`WasmExecutor`](sc_executor::WasmExecutor) and set the | ||
| //! correct generic type. | ||
| //! | ||
| //! This example from the parachain-template shows a type definition that includes the correct | ||
| //! host functions. | ||
| #![doc = docify::embed!("../../templates/parachain/node/src/service.rs", wasm_executor)] | ||
| //! | ||
| //! > **Note:** | ||
| //! > | ||
| //! > If you see error `runtime requires function imports which are not present on the host: | ||
| //! > 'env:ext_storage_proof_size_storage_proof_size_version_1'`, it is likely | ||
| //! > that this step in the guide was not set up correctly. | ||
| //! | ||
| //! ## 2. Enable storage proof recording during import | ||
| //! | ||
| //! The reclaim mechanism reads the size of the currently recorded storage proof multiple times | ||
| //! during block authoring and block import. Proof recording during authoring is already enabled on | ||
| //! parachains. You must also ensure that storage proof recording is enabled during block import. | ||
| //! Find where your node builds the fundamental substrate components by calling | ||
| //! [`new_full_parts`](sc_service::new_full_parts). Replace this | ||
| //! with [`new_full_parts_record_import`](sc_service::new_full_parts_record_import) and | ||
| //! pass `true` as the last parameter to enable import recording. | ||
| #![doc = docify::embed!("../../templates/parachain/node/src/service.rs", component_instantiation)] | ||
| //! | ||
| //! > **Note:** | ||
| //! > | ||
| //! > If you see error `Storage root must match that calculated.` during block import, it is likely | ||
| //! > that this step in the guide was not | ||
| //! > set up correctly. | ||
| //! | ||
| //! ## 3. Add the SignedExtension to your runtime | ||
| //! | ||
| //! In your runtime, you will find a list of SignedExtensions. | ||
| //! To enable the reclaiming, | ||
| //! add [`StorageWeightReclaim`](cumulus_primitives_storage_weight_reclaim::StorageWeightReclaim) | ||
| //! to that list. For maximum efficiency, make sure that `StorageWeightReclaim` is last in the list. | ||
| //! The extension will check the size of the storage proof before and after an extrinsic execution. | ||
| //! It reclaims the difference between the calculated size and the benchmarked size. | ||
| #![doc = docify::embed!("../../templates/parachain/runtime/src/lib.rs", template_signed_extra)] | ||
| //! | ||
| //! ## Optional: Verify that reclaim works | ||
| //! | ||
| //! Start your node with the log target `runtime::storage_reclaim` set to `trace` to enable full | ||
| //! logging for `StorageWeightReclaim`. The following log is an example from a local testnet. To | ||
| //! trigger the log, execute any extrinsic on the network. | ||
| //! | ||
| //! ```ignore | ||
| //! ... | ||
| //! 2024-04-22 17:31:48.014 TRACE runtime::storage_reclaim: [ferdie] Reclaiming storage weight. benchmarked: 3593, consumed: 265 unspent: 0 | ||
| //! ... | ||
| //! ``` | ||
| //! | ||
| //! In the above example we see a benchmarked size of 3593 bytes, while the extrinsic only consumed | ||
| //! 265 bytes of proof size. This results in 3328 bytes of reclaim. | ||
| #![deny(rustdoc::broken_intra_doc_links)] | ||
| #![deny(rustdoc::private_intra_doc_links)] |
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