Full node with no validators attached to store only custody columns and discard extra sampling columns

[jimmygchen]

Description

When subnet sampling was implemented in #6410, I didn't implement discarding the extra sampling columns (see comment) for the reasons below

• It wasn't clear whether we were going to implement peer sampling in Fulu at the time, which would make the change redundant
• CustodyContext didn't exist and there wasn't an simple and clean way for the beacon chain to filter out sampling columns
  

  lighthouse/beacon_node/beacon_chain/src/validator_custody.rs

  Line 145 in 3de646c

  all_custody_columns_ordered: OnceLock<Box<[ColumnIndex]>>,

So there was a bit of extra effort with some chance that it would be eventually reverted - and even with the change, the benefit for full nodes was quite minimal - they would still have to download & sample the 4 extra columns, and saving only 64kb for a 8-blob block.

However, this seems worth implementing now peer sampling has been completely removed from the spec, and also this is now straight forward to do because BeaconChain knows exact which columns to discard.

[jimmygchen]

Note that this will be a backward incompatible change for testnet nodes with fulu activated.

[michaelsproul]

Closed by:

• Persist only custody columns in db #8188

[chong-he]

Unpin this issue as it is closed