What should be the maximum size of a `BoundedVec`?

Question

What is the recommended maximum number of bytes to store within a BoundedVec?

I am currently having the following struct:

#[derive(
    parity_scale_codec::Decode,
    parity_scale_codec::Encode,
    parity_scale_codec::MaxEncodedLen,
    scale_info::TypeInfo,
    Clone,
    Debug,
    PartialEq,
    Eq,
)]
pub struct JurorPoolItem<AccountId, Balance> {
    pub(crate) stake: Balance,
    pub(crate) juror: AccountId,
    pub(crate) consumed_stake: Balance,
}

And a BoundedVec:

pub(crate) type JurorPoolItemOf<T> = JurorPoolItem<AccountIdOf<T>, BalanceOf<T>>;
pub(crate) type JurorPoolOf<T> = BoundedVec<JurorPoolItemOf<T>, <T as Config>::MaxJurors>;

With the config parameter:

pub const MaxJurors: u32 = 10_000;

Is that too much for a block to handle if I iterate through this for example three times?

Answer 1

Is that too much for a block to handle if I iterate through this for example three times?

The answer to this question is benchmarking, the result of running such benchmarks will give a value that you can use to decide if the cost of those operations are bigger than expected.

Here a few links on benchmarking:

• Benchmark FRAME pallets
• https://docs.substrate.io/test/benchmark/
• https://docs.substrate.io/reference/how-to-guides/weights/add-benchmarks/
• https://github.com/paritytech/substrate/tree/master/utils/frame/benchmarking-cli

What is the recommended maximum number of bytes to store within a BoundedVec?

For this questions my understanding is that, when working with Wasm, 32bit, you can at maximum have an array of u32 max elements.

Answer 2

Just to add one thing into Alejandro's answer, the new Weights V2 has added a field into Weights: proof_size:

    pub struct Weight {
      /// The weight of computational time used based on 
      /// some reference hardware.
       ref_time: u64,
      /// The weight of storage space used by proof of validity.
      proof_size: u64,
    }

So the Weights now are not only measuring the computational time used, is measuring the storage space used by proof of validity.

In case you have a parachain, the proper parameter to set for the max proof size comes from the relay chain. In Polkadot you can see it here:

    pub const MAX_POV_SIZE: u32 = 5 * 1024 * 1024;