Pre-allocate vdev I/O buffers

The vdev queue layer may require a small number of buffers
when attempting to create aggregate I/O requests.  Rather than
attempting to allocate them from the global zio buffers, which
is slow under memory pressure, it makes sense to pre-allocate
them because...

1) These buffers are short lived.  They are only required for
the life of a single I/O at which point they can be used by
the next I/O.

2) The maximum number of concurrent buffers needed by a vdev is
small.  It's roughly limited by the zfs_vdev_max_pending tunable
which defaults to 10.

By keeping a small list of these buffer per-vdev we can ensure
one is always available when we need it.  This significantly
reduces contention on the vq->vq_lock, because we no longer
need to perform a slow allocation under this lock.  This is
particularly important when memory is already low on the system.

It would probably be wise to extend the use of these buffers beyond
aggregate I/O and in to the raidz implementation.  The inability
to quickly allocate buffer for the parity stripes could result in
similiar problems.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

include/sys/vdev_impl.h

@@ -49,6 +49,7 @@ extern "C" {
  * Forward declarations that lots of things need.
  */
 typedef struct vdev_queue vdev_queue_t;
+typedef struct vdev_io vdev_io_t;
 typedef struct vdev_cache vdev_cache_t;
 typedef struct vdev_cache_entry vdev_cache_entry_t;
 
@@ -102,9 +103,15 @@ struct vdev_queue {
 	avl_tree_t	vq_read_tree;
 	avl_tree_t	vq_write_tree;
 	avl_tree_t	vq_pending_tree;
+	list_t		vq_io_list;
 	kmutex_t	vq_lock;
 };
 
+struct vdev_io {
+	char		vi_buffer[SPA_MAXBLOCKSIZE]; /* Must be first */
+	list_node_t	vi_node;
+};
+
 /*
  * Virtual device descriptor
  */

include/sys/zio.h

@@ -494,6 +494,8 @@ extern void *zio_buf_alloc(size_t size);
 extern void zio_buf_free(void *buf, size_t size);
 extern void *zio_data_buf_alloc(size_t size);
 extern void zio_data_buf_free(void *buf, size_t size);
+extern void *zio_vdev_alloc(void);
+extern void zio_vdev_free(void *buf);
 
 extern void zio_resubmit_stage_async(void *);