During pool export flush the ARC asynchronously

This also includes removing L2 vdevs asynchronously

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.

Signed-off-by: Don Brady <don.brady@klarasystems.com>

include/sys/arc.h

@@ -63,8 +63,15 @@ extern "C" {
 	(hdr)->b_psize = ((x) >> SPA_MINBLOCKSHIFT); \
 } while (0)
 
+/* The asize in the header is only used by L2 cache */
+#define	HDR_SET_ASIZE(hdr, x) do { \
+	ASSERT(IS_P2ALIGNED((x), 1U << SPA_MINBLOCKSHIFT)); \
+	(hdr)->b_asize = ((x) >> SPA_MINBLOCKSHIFT); \
+} while (0)
+
 #define	HDR_GET_LSIZE(hdr)	((hdr)->b_lsize << SPA_MINBLOCKSHIFT)
 #define	HDR_GET_PSIZE(hdr)	((hdr)->b_psize << SPA_MINBLOCKSHIFT)
+#define	HDR_GET_ASIZE(hdr)	((hdr)->b_asize << SPA_MINBLOCKSHIFT)
 
 typedef struct arc_buf_hdr arc_buf_hdr_t;
 typedef struct arc_buf arc_buf_t;
@@ -312,6 +319,7 @@ void arc_remove_prune_callback(arc_prune_t *p);
 void arc_freed(spa_t *spa, const blkptr_t *bp);
 
 void arc_flush(spa_t *spa, boolean_t retry);
+void arc_flush_async(spa_t *spa);
 void arc_tempreserve_clear(uint64_t reserve);
 int arc_tempreserve_space(spa_t *spa, uint64_t reserve, uint64_t txg);
 

include/sys/arc_impl.h

@@ -378,8 +378,8 @@ typedef struct l2arc_lb_ptr_buf {
  * L2ARC Internals
  */
 typedef struct l2arc_dev {
-	vdev_t			*l2ad_vdev;	/* vdev */
-	spa_t			*l2ad_spa;	/* spa */
+	vdev_t			*l2ad_vdev;	/* can be NULL during remove */
+	spa_t			*l2ad_spa;	/* can be NULL during remove */
 	uint64_t		l2ad_hand;	/* next write location */
 	uint64_t		l2ad_start;	/* first addr on device */
 	uint64_t		l2ad_end;	/* last addr on device */
@@ -475,8 +475,8 @@ struct arc_buf_hdr {
 
 	arc_buf_contents_t	b_type;
 	uint8_t			b_complevel;
-	uint8_t			b_reserved1; /* used for 4 byte alignment */
-	uint16_t		b_reserved2; /* used for 4 byte alignment */
+	uint8_t			b_reserved1;	/* used for 4 byte alignment */
+	uint16_t		b_asize;	/* alignment or L2-only asize */
 	arc_buf_hdr_t		*b_hash_next;
 	arc_flags_t		b_flags;