• Add the B_NEEDSGIANT flag which is only set if the vnode that owns a buf requires Giant. It is set in bgetvp and cleared in brelvp.
• Create QUEUE_DIRTY_GIANT for dirty buffers that require giant.
• In the buf daemon, only grab giant when processing QUEUE_DIRTY_GIANT and only if we think there are buffers in that queue.

Sponsored by: Isilon Systems, Inc.