--- a/usr/src/cmd/mdb/common/modules/zfs/zfs.c	Thu Sep 27 13:58:00 2012 -0400
+++ b/usr/src/cmd/mdb/common/modules/zfs/zfs.c	Thu Sep 27 21:55:09 2012 -0700
@@ -905,7 +905,8 @@
 	const char *suffix;
 
 	static const char *bytestats[] = {
-		"p", "c", "c_min", "c_max", "size", NULL
+		"p", "c", "c_min", "c_max", "size", "duplicate_buffers_size",
+		NULL
 	};
 
 	static const char *extras[] = {

--- a/usr/src/cmd/ztest/ztest.c	Thu Sep 27 13:58:00 2012 -0400
+++ b/usr/src/cmd/ztest/ztest.c	Thu Sep 27 21:55:09 2012 -0700
@@ -4695,7 +4695,18 @@
 			if (islog)
 				(void) rw_unlock(&ztest_name_lock);
 		} else {
+			/*
+			 * Ideally we would like to be able to randomly
+			 * call vdev_[on|off]line without holding locks
+			 * to force unpredictable failures but the side
+			 * effects of vdev_[on|off]line prevent us from
+			 * doing so. We grab the ztest_vdev_lock here to
+			 * prevent a race between injection testing and
+			 * aux_vdev removal.
+			 */
+			VERIFY(mutex_lock(&ztest_vdev_lock) == 0);
 			(void) vdev_online(spa, guid0, 0, NULL);
+			VERIFY(mutex_unlock(&ztest_vdev_lock) == 0);
 		}
 	}
 

--- a/usr/src/uts/common/fs/zfs/arc.c	Thu Sep 27 13:58:00 2012 -0400
+++ b/usr/src/uts/common/fs/zfs/arc.c	Thu Sep 27 21:55:09 2012 -0700
@@ -188,6 +188,7 @@
 int zfs_arc_grow_retry = 0;
 int zfs_arc_shrink_shift = 0;
 int zfs_arc_p_min_shift = 0;
+int zfs_disable_dup_eviction = 0;
 
 /*
  * Note that buffers can be in one of 6 states:
@@ -290,6 +291,9 @@
 	kstat_named_t arcstat_l2_size;
 	kstat_named_t arcstat_l2_hdr_size;
 	kstat_named_t arcstat_memory_throttle_count;
+	kstat_named_t arcstat_duplicate_buffers;
+	kstat_named_t arcstat_duplicate_buffers_size;
+	kstat_named_t arcstat_duplicate_reads;
 } arc_stats_t;
 
 static arc_stats_t arc_stats = {
@@ -345,7 +349,10 @@
 	{ "l2_io_error",		KSTAT_DATA_UINT64 },
 	{ "l2_size",			KSTAT_DATA_UINT64 },
 	{ "l2_hdr_size",		KSTAT_DATA_UINT64 },
-	{ "memory_throttle_count",	KSTAT_DATA_UINT64 }
+	{ "memory_throttle_count",	KSTAT_DATA_UINT64 },
+	{ "duplicate_buffers",		KSTAT_DATA_UINT64 },
+	{ "duplicate_buffers_size",	KSTAT_DATA_UINT64 },
+	{ "duplicate_reads",		KSTAT_DATA_UINT64 }
 };
 
 #define	ARCSTAT(stat)	(arc_stats.stat.value.ui64)
@@ -1360,6 +1367,17 @@
 	hdr->b_buf = buf;
 	arc_get_data_buf(buf);
 	bcopy(from->b_data, buf->b_data, size);
+
+	/*
+	 * This buffer already exists in the arc so create a duplicate
+	 * copy for the caller.  If the buffer is associated with user data
+	 * then track the size and number of duplicates.  These stats will be
+	 * updated as duplicate buffers are created and destroyed.
+	 */
+	if (hdr->b_type == ARC_BUFC_DATA) {
+		ARCSTAT_BUMP(arcstat_duplicate_buffers);
+		ARCSTAT_INCR(arcstat_duplicate_buffers_size, size);
+	}
 	hdr->b_datacnt += 1;
 	return (buf);
 }
@@ -1458,6 +1476,16 @@
 		ASSERT3U(state->arcs_size, >=, size);
 		atomic_add_64(&state->arcs_size, -size);
 		buf->b_data = NULL;
+
+		/*
+		 * If we're destroying a duplicate buffer make sure
+		 * that the appropriate statistics are updated.
+		 */
+		if (buf->b_hdr->b_datacnt > 1 &&
+		    buf->b_hdr->b_type == ARC_BUFC_DATA) {
+			ARCSTAT_BUMPDOWN(arcstat_duplicate_buffers);
+			ARCSTAT_INCR(arcstat_duplicate_buffers_size, -size);
+		}
 		ASSERT(buf->b_hdr->b_datacnt > 0);
 		buf->b_hdr->b_datacnt -= 1;
 	}
@@ -1642,6 +1670,48 @@
 }
 
 /*
+ * Called from the DMU to determine if the current buffer should be
+ * evicted. In order to ensure proper locking, the eviction must be initiated
+ * from the DMU. Return true if the buffer is associated with user data and
+ * duplicate buffers still exist.
+ */
+boolean_t
+arc_buf_eviction_needed(arc_buf_t *buf)
+{
+	arc_buf_hdr_t *hdr;
+	boolean_t evict_needed = B_FALSE;
+
+	if (zfs_disable_dup_eviction)
+		return (B_FALSE);
+
+	mutex_enter(&buf->b_evict_lock);
+	hdr = buf->b_hdr;
+	if (hdr == NULL) {
+		/*
+		 * We are in arc_do_user_evicts(); let that function
+		 * perform the eviction.
+		 */
+		ASSERT(buf->b_data == NULL);
+		mutex_exit(&buf->b_evict_lock);
+		return (B_FALSE);
+	} else if (buf->b_data == NULL) {
+		/*
+		 * We have already been added to the arc eviction list;
+		 * recommend eviction.
+		 */
+		ASSERT3P(hdr, ==, &arc_eviction_hdr);
+		mutex_exit(&buf->b_evict_lock);
+		return (B_TRUE);
+	}
+
+	if (hdr->b_datacnt > 1 && hdr->b_type == ARC_BUFC_DATA)
+		evict_needed = B_TRUE;
+
+	mutex_exit(&buf->b_evict_lock);
+	return (evict_needed);
+}
+
+/*
  * Evict buffers from list until we've removed the specified number of
  * bytes.  Move the removed buffers to the appropriate evict state.
  * If the recycle flag is set, then attempt to "recycle" a buffer:
@@ -2626,8 +2696,10 @@
 	abuf = buf;
 	for (acb = callback_list; acb; acb = acb->acb_next) {
 		if (acb->acb_done) {
-			if (abuf == NULL)
+			if (abuf == NULL) {
+				ARCSTAT_BUMP(arcstat_duplicate_reads);
 				abuf = arc_buf_clone(buf);
+			}
 			acb->acb_buf = abuf;
 			abuf = NULL;
 		}
@@ -3166,6 +3238,16 @@
 			ASSERT3U(*size, >=, hdr->b_size);
 			atomic_add_64(size, -hdr->b_size);
 		}
+
+		/*
+		 * We're releasing a duplicate user data buffer, update
+		 * our statistics accordingly.
+		 */
+		if (hdr->b_type == ARC_BUFC_DATA) {
+			ARCSTAT_BUMPDOWN(arcstat_duplicate_buffers);
+			ARCSTAT_INCR(arcstat_duplicate_buffers_size,
+			    -hdr->b_size);
+		}
 		hdr->b_datacnt -= 1;
 		arc_cksum_verify(buf);
 		arc_buf_unwatch(buf);

--- a/usr/src/uts/common/fs/zfs/dbuf.c	Thu Sep 27 13:58:00 2012 -0400
+++ b/usr/src/uts/common/fs/zfs/dbuf.c	Thu Sep 27 21:55:09 2012 -0700
@@ -2089,7 +2089,24 @@
 			dbuf_evict(db);
 		} else {
 			VERIFY(arc_buf_remove_ref(db->db_buf, db) == 0);
-			if (!DBUF_IS_CACHEABLE(db))
+
+			/*
+			 * A dbuf will be eligible for eviction if either the
+			 * 'primarycache' property is set or a duplicate
+			 * copy of this buffer is already cached in the arc.
+			 *
+			 * In the case of the 'primarycache' a buffer
+			 * is considered for eviction if it matches the
+			 * criteria set in the property.
+			 *
+			 * To decide if our buffer is considered a
+			 * duplicate, we must call into the arc to determine
+			 * if multiple buffers are referencing the same
+			 * block on-disk. If so, then we simply evict
+			 * ourselves.
+			 */
+			if (!DBUF_IS_CACHEABLE(db) ||
+			    arc_buf_eviction_needed(db->db_buf))
 				dbuf_clear(db);
 			else
 				mutex_exit(&db->db_mtx);

--- a/usr/src/uts/common/fs/zfs/sys/arc.h	Thu Sep 27 13:58:00 2012 -0400
+++ b/usr/src/uts/common/fs/zfs/sys/arc.h	Thu Sep 27 21:55:09 2012 -0700
@@ -99,6 +99,7 @@
 int arc_has_callback(arc_buf_t *buf);
 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);
 #ifdef ZFS_DEBUG
 int arc_referenced(arc_buf_t *buf);
 #endif