Function resolve_arena_bytes_from 

pub fn resolve_arena_bytes_from(
    raw: Option<&str>,
    block_bytes: u64,
) -> Result<u64>

ATLAS_KV_PAGING_ARENA_GB (REQUIRED when the flag is on — no implicit default, PCND): the peer warm-arena size in GiB (fractional accepted), floored to a multiple of block_bytes and required to hold ≥ 1 block. The raw path sized the peer to num_groups × group_stride (every group a guaranteed slot); the paging arena is a deliberately smaller warm cache over the peer’s NVMe swap, so the operator must choose it explicitly.