Constant READ_PREFETCH_BYTES 

const READ_PREFETCH_BYTES: usize = _; // 67_108_864usize

Bytes of decoded segments that all reads of a store together fetch ahead; a segment counts as its length, which bounds its payloads, plus a Record per entry. A read always fetches the segment it yields next, as a read that fetched one segment at a time did, and a later segment only while it fits in what is left, so regions replayed in parallel hold one segment each, up to about max_batch_bytes or more for one oversized append, plus at most these bytes, however many regions open at once. While a fetch copies the payloads out of the downloaded segment it briefly holds the segment’s length once more.