7843 get_clones_stat() is suboptimal for lots of clones

Reviewed by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Dan McDonald <danmcd@omniti.com>

usr/src/uts/common/fs/zfs/dsl_dataset.c

@@ -1687,10 +1687,21 @@ get_clones_stat(dsl_dataset_t *ds, nvlist_t *nv)
 	zap_cursor_t zc;
 	zap_attribute_t za;
 	nvlist_t *propval = fnvlist_alloc();
-	nvlist_t *val = fnvlist_alloc();
+	nvlist_t *val;
 
 	ASSERT(dsl_pool_config_held(ds->ds_dir->dd_pool));
 
+	/*
+	 * We use nvlist_alloc() instead of fnvlist_alloc() because the
+	 * latter would allocate the list with NV_UNIQUE_NAME flag.
+	 * As a result, every time a clone name is appended to the list
+	 * it would be (linearly) searched for for a duplicate name.
+	 * We already know that all clone names must be unique and we
+	 * want avoid the quadratic complexity of double-checking that
+	 * because we can have a large number of clones.
+	 */
+	VERIFY0(nvlist_alloc(&val, 0, KM_SLEEP));
+
 	/*
 	 * There may be missing entries in ds_next_clones_obj
 	 * due to a bug in a previous version of the code.