Index: spa.c
===================================================================
--- spa.c	(revision 203410)
+++ spa.c	(working copy)
@@ -58,20 +58,24 @@
 #include <sys/callb.h>
 #include <sys/sunddi.h>
 #include <sys/spa_boot.h>
+#include <sys/smp.h>
 
 #include "zfs_prop.h"
 #include "zfs_comutil.h"
 
-int zio_taskq_threads[ZIO_TYPES][ZIO_TASKQ_TYPES] = {
+static int zio_taskq_threads[ZIO_TYPES][ZIO_TASKQ_TYPES] = {
 	/*	ISSUE	INTR					*/
 	{	1,	1	},	/* ZIO_TYPE_NULL	*/
-	{	1,	8	},	/* ZIO_TYPE_READ	*/
-	{	8,	1	},	/* ZIO_TYPE_WRITE	*/
+	{	1,	-1	},	/* ZIO_TYPE_READ	*/
+	{	-1,	1	},	/* ZIO_TYPE_WRITE	*/
 	{	1,	1	},	/* ZIO_TYPE_FREE	*/
 	{	1,	1	},	/* ZIO_TYPE_CLAIM	*/
 	{	1,	1	},	/* ZIO_TYPE_IOCTL	*/
 };
 
+static int zio_worker_threads_count = -1;
+TUNABLE_INT("vfs.zfs.zio_worker_threads_count", &zio_worker_threads_count);
+
 static void spa_sync_props(void *arg1, void *arg2, cred_t *cr, dmu_tx_t *tx);
 static boolean_t spa_has_active_shared_spare(spa_t *spa);
 
@@ -498,7 +502,11 @@ spa_activate(spa_t *spa)
 	for (int t = 0; t < ZIO_TYPES; t++) {
 		for (int q = 0; q < ZIO_TASKQ_TYPES; q++) {
 			spa->spa_zio_taskq[t][q] = taskq_create("spa_zio",
-			    zio_taskq_threads[t][q], maxclsyspri, 50,
+			    zio_taskq_threads[t][q] == -1 ? 
+			        (zio_worker_threads_count == -1 ? mp_ncpus :
+				     zio_worker_threads_count) :
+				zio_taskq_threads[t][q],
+			    maxclsyspri, 50,
 			    INT_MAX, TASKQ_PREPOPULATE);
 		}
 	}