The transaction log is stored sequentially on disk, filling extents in each allocation unit. Each time the recovery process reads a log page from a new allocation unit, it prefetches all the pages on that allocation unit that are in use by the log.
In databases that do not have a separate log segment, log and data extents may be mixed on the same allocation unit. Asynchronous prefetch still fetches all the log pages on the allocation unit, but the look-ahead sets may be smaller.
