Checkpoints and so on

onmode -B does not flush the pages from the buffer pool. It simply
writes dirty pages to disk. The page still remains in the buffer pool.
onmode -B is probably a much better solution to the long checkpoint
issue than using very low LRU values because it uses chunk writes
rather than LRU writes.
The engine uses two basic approches to writing dirty pages to disk.
The chunk writes that are used during the checkpoint (and onmode -B)
are sorted and organized arround the physical chunk. Because of this,
they are are rather efficient -- sort of like using scatter-gather IO.
The LRU writes are not organized and thus not nearly as efficient.