Index: sys/cddl/contrib/opensolaris/uts/common/fs/zfs/spa_misc.c
===================================================================
--- sys/cddl/contrib/opensolaris/uts/common/fs/zfs/spa_misc.c	(revision 354667)
+++ sys/cddl/contrib/opensolaris/uts/common/fs/zfs/spa_misc.c	(working copy)
@@ -373,7 +373,7 @@ SYSCTL_UQUAD(_vfs_zfs, OID_AUTO, deadman
 SYSCTL_UQUAD(_vfs_zfs, OID_AUTO, deadman_checktime_ms, CTLFLAG_RDTUN,
     &zfs_deadman_checktime_ms, 0,
     "Period of checks for stalled ZFS I/O in milliseconds");
-SYSCTL_INT(_vfs_zfs, OID_AUTO, deadman_enabled, CTLFLAG_RDTUN,
+SYSCTL_INT(_vfs_zfs, OID_AUTO, deadman_enabled, CTLFLAG_RWTUN,
     &zfs_deadman_enabled, 0, "Kernel panic on stalled ZFS I/O");
 SYSCTL_INT(_vfs_zfs, OID_AUTO, spa_asize_inflation, CTLFLAG_RWTUN,
     &spa_asize_inflation, 0, "Worst case inflation factor for single sector writes");
Index: sys/cddl/contrib/opensolaris/uts/common/fs/zfs/vdev.c
===================================================================
--- sys/cddl/contrib/opensolaris/uts/common/fs/zfs/vdev.c	(revision 354667)
+++ sys/cddl/contrib/opensolaris/uts/common/fs/zfs/vdev.c	(working copy)
@@ -29,6 +29,8 @@
  * Copyright 2017 Joyent, Inc.
  */
 
+#include <sys/syslog.h>
+
 #include <sys/zfs_context.h>
 #include <sys/fm/fs/zfs.h>
 #include <sys/spa.h>
@@ -4336,6 +4338,8 @@ vdev_split(vdev_t *vd)
 	vdev_propagate_state(cvd);
 }
 
+extern int zfs_deadman_enabled;
+
 void
 vdev_deadman(vdev_t *vd)
 {
@@ -4347,12 +4351,13 @@ vdev_deadman(vdev_t *vd)
 
 	if (vd->vdev_ops->vdev_op_leaf) {
 		vdev_queue_t *vq = &vd->vdev_queue;
+		ulong_t num;
 
 		mutex_enter(&vq->vq_lock);
-		if (avl_numnodes(&vq->vq_active_tree) > 0) {
+		if ((num = avl_numnodes(&vq->vq_active_tree)) > 0) {
 			spa_t *spa = vd->vdev_spa;
 			zio_t *fio;
-			uint64_t delta;
+			uint64_t delta, early;
 
 			/*
 			 * Look at the head of all the pending queues,
@@ -4361,11 +4366,28 @@ vdev_deadman(vdev_t *vd)
 			 */
 			fio = avl_first(&vq->vq_active_tree);
 			delta = gethrtime() - fio->io_timestamp;
+			early = delta / 8;
+
+			if (zfs_deadman_enabled > 1 &&
+			    early > spa_deadman_synctime(spa)) {
+				log(LOG_ERR, "%s SLOW IO: zio io_type %u "
+				    "timestamp %lluns, delta/8 %lluns, "
+				    "last io %lluns",
+				    spa_name(spa), fio->io_type,
+				    fio->io_timestamp, (u_longlong_t)early,
+				    vq->vq_io_complete_ts);
+				log(LOG_ERR, "I/O to pool '%s' appears to be "
+				    "hung on vdev guid %llu at '%s' active zio %lu.",
+				    spa_name(spa),
+				    (long long unsigned int) vd->vdev_guid,
+				    vd->vdev_path, num);
+			} else
 			if (delta > spa_deadman_synctime(spa)) {
 				vdev_dbgmsg(vd, "SLOW IO: zio timestamp "
 				    "%lluns, delta %lluns, last io %lluns",
 				    fio->io_timestamp, (u_longlong_t)delta,
 				    vq->vq_io_complete_ts);
+			    if(zfs_deadman_enabled == 1)
 				fm_panic("I/O to pool '%s' appears to be "
 				    "hung on vdev guid %llu at '%s'.",
 				    spa_name(spa),