Constant DURABLE 

pub const DURABLE: u8 = 111;

Log format 3: shards’ durable positions after a batch, cumulative (a later frame never lowers one): id u64 (0), position u64, segment u64, count u32, then per shard: shard u32, lsn u64, flags u8 (bit 0: the shard alone holds back retiring the oldest log segment). position is the next batch’s sequence and segment the active one, for checkpoints.