I want to spin up parachain LOCALLY faster. each 6 sec. How to configure polkadot with parachain?

I have seen this and this and this post

So my zombienet contains polkadot(obviusly😊) and two parachains. each of them with one collator node.

I changed relaychain

pub const MILLISECS_PER_BLOCK: u64 = 3000;

and now relay chain produce the block each 3 sec. it is good.

enter image description here

I changed for parachain runtime. the value. from 12000 to 6000

pub const MILLISECS_PER_BLOCK: u64 = 6000;

BUT parachain still produce the block each 12 sec instead of 6 even when relay chain produce block each 3 sec.

I understand that i need to reconfigure relay chain to sign and send transaction to parachain more often. set_validate_data extrinisc in parachain system

enter image description here


2 Answers 2


If you want to run the relay chain and parachain locally follow https://docs.substrate.io/tutorials/build-a-parachain/prepare-a-local-relay-chain/.

The above tutorial will help you set up a relay chain locally and also will tell you how to connect a local parachain to it, after setting up the chain you can configure it according to your needs.

  • thanks. i aware about how to run locally. i am interested how to configure relaychain + parachain to produce block faster for parachain. at least every 6 sec instead of 12 sec.
    – rust.dev
    May 18, 2023 at 9:31
  • 1
    it should have worked by changing the MILLISECS_PER_BLOCK, time also doesn't changed on parachain's explorer as well? May 18, 2023 at 10:20
  • Once i changed MILLISECS_PER_BLOCK for parachain runtime it does not work. Still 12 sec per block instead of 6. even when relay chain produces the block each 3 sec.
    – rust.dev
    May 18, 2023 at 10:39

polkadot/service/chain_spec.rs will override


So you do not need change MILLISECS_PER_BLOCK directly. You need to change


#[cfg(feature = "rococo-native")]
pub fn rococo_development_config() -> Result<RococoChainSpec, String> {
    let wasm_binary = rococo::WASM_BINARY.ok_or("Rococo development wasm not available")?;

            move || RococoGenesisExt {
                runtime_genesis_config: rococo_development_config_genesis(wasm_binary),
                // Use 1 minute session length.
                session_length_in_blocks: Some(10),

Because the value of MILLISECS_PER_BLOCK will be overided byt this code

#[cfg(feature = "rococo-native")]
impl sp_runtime::BuildStorage for RococoGenesisExt {
    fn assimilate_storage(&self, storage: &mut sp_core::storage::Storage) -> Result<(), String> {
        sp_state_machine::BasicExternalities::execute_with_storage(storage, || {
            if let Some(length) = self.session_length_in_blocks.as_ref() {

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