diff --git a/cmd/ztest/ztest.c b/cmd/ztest/ztest.c
index 7b09532c0..3b26f8d1e 100644
--- a/cmd/ztest/ztest.c
+++ b/cmd/ztest/ztest.c
@@ -6210,11 +6210,12 @@ main(int argc, char **argv)
 
 	(void) setvbuf(stdout, NULL, _IOLBF, 0);
 
+	dprintf_setup(&argc, argv);
+
 	ztest_fd_rand = open("/dev/urandom", O_RDONLY);
 	ASSERT3S(ztest_fd_rand, >=, 0);
 
 	if (!fd_data_str) {
-		dprintf_setup(&argc, argv);
 		process_options(argc, argv);
 
 		setup_data_fd();
diff --git a/include/sys/arc.h b/include/sys/arc.h
index efafb551a..221946da3 100644
--- a/include/sys/arc.h
+++ b/include/sys/arc.h
@@ -136,6 +136,7 @@ int arc_buf_size(arc_buf_t *buf);
 void arc_release(arc_buf_t *buf, void *tag);
 int arc_released(arc_buf_t *buf);
 int arc_has_callback(arc_buf_t *buf);
+void arc_buf_sigsegv(int sig, siginfo_t *si, void *unused);
 void arc_buf_freeze(arc_buf_t *buf);
 void arc_buf_thaw(arc_buf_t *buf);
 boolean_t arc_buf_eviction_needed(arc_buf_t *buf);
@@ -183,6 +184,10 @@ extern int zfs_write_limit_shift;
 extern unsigned long zfs_write_limit_max;
 extern kmutex_t zfs_write_limit_lock;
 
+#ifndef _KERNEL
+extern boolean_t arc_watch;
+#endif
+
 #ifdef	__cplusplus
 }
 #endif
diff --git a/include/sys/zfs_context.h b/include/sys/zfs_context.h
index adb152f58..28a1306aa 100644
--- a/include/sys/zfs_context.h
+++ b/include/sys/zfs_context.h
@@ -97,6 +97,8 @@
 #include <dirent.h>
 #include <time.h>
 #include <ctype.h>
+#include <signal.h>
+#include <sys/mman.h>
 #include <sys/note.h>
 #include <sys/types.h>
 #include <sys/cred.h>
diff --git a/module/zfs/arc.c b/module/zfs/arc.c
index 04f87a810..5bba9a317 100644
--- a/module/zfs/arc.c
+++ b/module/zfs/arc.c
@@ -145,6 +145,11 @@
 #include <sys/dmu_tx.h>
 #include <zfs_fletcher.h>
 
+#ifndef _KERNEL
+/* set with ZFS_DEBUG=watch, to enable watchpoints on frozen buffers */
+boolean_t arc_watch = B_FALSE;
+#endif
+
 static kmutex_t		arc_reclaim_thr_lock;
 static kcondvar_t	arc_reclaim_thr_cv;	/* used to signal reclaim thr */
 static uint8_t		arc_thread_exit;
@@ -569,6 +574,7 @@ static void arc_access(arc_buf_hdr_t *buf, kmutex_t *hash_lock);
 static int arc_evict_needed(arc_buf_contents_t type);
 static void arc_evict_ghost(arc_state_t *state, uint64_t spa, int64_t bytes,
     arc_buf_contents_t type);
+static void arc_buf_watch(arc_buf_t *buf);
 
 static boolean_t l2arc_write_eligible(uint64_t spa_guid, arc_buf_hdr_t *ab);
 
@@ -1060,6 +1066,37 @@ arc_cksum_compute(arc_buf_t *buf, boolean_t force)
 	fletcher_2_native(buf->b_data, buf->b_hdr->b_size,
 	    buf->b_hdr->b_freeze_cksum);
 	mutex_exit(&buf->b_hdr->b_freeze_lock);
+	arc_buf_watch(buf);
+}
+
+#ifndef _KERNEL
+void
+arc_buf_sigsegv(int sig, siginfo_t *si, void *unused)
+{
+	panic("Got SIGSEGV at address: 0x%lx\n", (long) si->si_addr);
+}
+#endif
+
+/* ARGSUSED */
+static void
+arc_buf_unwatch(arc_buf_t *buf)
+{
+#ifndef _KERNEL
+	if (arc_watch) {
+		ASSERT0(mprotect(buf->b_data, buf->b_hdr->b_size,
+		    PROT_READ | PROT_WRITE));
+	}
+#endif
+}
+
+/* ARGSUSED */
+static void
+arc_buf_watch(arc_buf_t *buf)
+{
+#ifndef _KERNEL
+	if (arc_watch)
+		ASSERT0(mprotect(buf->b_data, buf->b_hdr->b_size, PROT_READ));
+#endif
 }
 
 void
@@ -1080,6 +1117,8 @@ arc_buf_thaw(arc_buf_t *buf)
 	}
 
 	mutex_exit(&buf->b_hdr->b_freeze_lock);
+
+	arc_buf_unwatch(buf);
 }
 
 void
@@ -1097,6 +1136,7 @@ arc_buf_freeze(arc_buf_t *buf)
 	    buf->b_hdr->b_state == arc_anon);
 	arc_cksum_compute(buf, B_FALSE);
 	mutex_exit(hash_lock);
+
 }
 
 static void
@@ -1504,21 +1544,22 @@ arc_buf_add_ref(arc_buf_t *buf, void* tag)
  * the buffer is placed on l2arc_free_on_write to be freed later.
  */
 static void
-arc_buf_data_free(arc_buf_hdr_t *hdr, void (*free_func)(void *, size_t),
-    void *data, size_t size)
+arc_buf_data_free(arc_buf_t *buf, void (*free_func)(void *, size_t))
 {
+	arc_buf_hdr_t *hdr = buf->b_hdr;
+
 	if (HDR_L2_WRITING(hdr)) {
 		l2arc_data_free_t *df;
 		df = kmem_alloc(sizeof (l2arc_data_free_t), KM_PUSHPAGE);
-		df->l2df_data = data;
-		df->l2df_size = size;
+		df->l2df_data = buf->b_data;
+		df->l2df_size = hdr->b_size;
 		df->l2df_func = free_func;
 		mutex_enter(&l2arc_free_on_write_mtx);
 		list_insert_head(l2arc_free_on_write, df);
 		mutex_exit(&l2arc_free_on_write_mtx);
 		ARCSTAT_BUMP(arcstat_l2_free_on_write);
 	} else {
-		free_func(data, size);
+		free_func(buf->b_data, hdr->b_size);
 	}
 }
 
@@ -1534,16 +1575,15 @@ arc_buf_destroy(arc_buf_t *buf, boolean_t recycle, boolean_t all)
 		arc_buf_contents_t type = buf->b_hdr->b_type;
 
 		arc_cksum_verify(buf);
+		arc_buf_unwatch(buf);
 
 		if (!recycle) {
 			if (type == ARC_BUFC_METADATA) {
-				arc_buf_data_free(buf->b_hdr, zio_buf_free,
-				    buf->b_data, size);
+				arc_buf_data_free(buf, zio_buf_free);
 				arc_space_return(size, ARC_SPACE_DATA);
 			} else {
 				ASSERT(type == ARC_BUFC_DATA);
-				arc_buf_data_free(buf->b_hdr,
-				    zio_data_buf_free, buf->b_data, size);
+				arc_buf_data_free(buf, zio_data_buf_free);
 				ARCSTAT_INCR(arcstat_data_size, -size);
 				atomic_add_64(&arc_size, -size);
 			}
@@ -2908,6 +2948,7 @@ arc_read_done(zio_t *zio)
 	}
 
 	arc_cksum_compute(buf, B_FALSE);
+	arc_buf_watch(buf);
 
 	if (hash_lock && zio->io_error == 0 && hdr->b_state == arc_anon) {
 		/*
@@ -3542,6 +3583,7 @@ arc_release(arc_buf_t *buf, void *tag)
 		}
 		hdr->b_datacnt -= 1;
 		arc_cksum_verify(buf);
+		arc_buf_unwatch(buf);
 
 		mutex_exit(hash_lock);
 
diff --git a/module/zfs/spa_misc.c b/module/zfs/spa_misc.c
index 7bdfbf738..91e7fdf35 100644
--- a/module/zfs/spa_misc.c
+++ b/module/zfs/spa_misc.c
@@ -1630,6 +1630,23 @@ spa_init(int mode)
 
 	spa_mode_global = mode;
 
+#ifndef _KERNEL
+	if (spa_mode_global != FREAD && dprintf_find_string("watch")) {
+		struct sigaction sa;
+
+		sa.sa_flags = SA_SIGINFO;
+		sigemptyset(&sa.sa_mask);
+		sa.sa_sigaction = arc_buf_sigsegv;
+
+		if (sigaction(SIGSEGV, &sa, NULL) == -1) {
+			perror("could not enable watchpoints: "
+			    "sigaction(SIGSEGV, ...) = ");
+		} else {
+			arc_watch = B_TRUE;
+		}
+	}
+#endif
+
 	fm_init();
 	refcount_init();
 	unique_init();
diff --git a/module/zfs/zio.c b/module/zfs/zio.c
index 1f803a44b..f0c9c0f08 100644
--- a/module/zfs/zio.c
+++ b/module/zfs/zio.c
@@ -169,11 +169,21 @@ zio_init(void)
 		while (p2 & (p2 - 1))
 			p2 &= p2 - 1;
 
+#ifndef _KERNEL
+		/*
+		 * If we are using watchpoints, put each buffer on its own page,
+		 * to eliminate the performance overhead of trapping to the
+		 * kernel when modifying a non-watched buffer that shares the
+		 * page with a watched buffer.
+		 */
+		if (arc_watch && !IS_P2ALIGNED(size, PAGESIZE))
+			continue;
+#endif
 		if (size <= 4 * SPA_MINBLOCKSIZE) {
 			align = SPA_MINBLOCKSIZE;
-		} else if (P2PHASE(size, PAGESIZE) == 0) {
+		} else if (IS_P2ALIGNED(size, PAGESIZE)) {
 			align = PAGESIZE;
-		} else if (P2PHASE(size, p2 >> 2) == 0) {
+		} else if (IS_P2ALIGNED(size, p2 >> 2)) {
 			align = p2 >> 2;
 		}