Index: sys/modules/zfs/Makefile
===================================================================
--- sys/modules/zfs/Makefile	(revision 195775)
+++ sys/modules/zfs/Makefile	(working copy)
@@ -63,8 +63,8 @@
 SRCS+=	${ZFS_SRCS}
 SRCS+=	vdev_geom.c
 
-# Use UMA for ZIO allocation. This is not stable.
-#CFLAGS+=-DZIO_USE_UMA
+# Use UMA for ZIO allocation.
+CFLAGS+=-DZIO_USE_UMA
 
 # Use FreeBSD's namecache.
 CFLAGS+=-DFREEBSD_NAMECACHE
Index: sys/cddl/contrib/opensolaris/uts/common/fs/zfs/arc.c
===================================================================
--- sys/cddl/contrib/opensolaris/uts/common/fs/zfs/arc.c	(revision 196083)
+++ sys/cddl/contrib/opensolaris/uts/common/fs/zfs/arc.c	(working copy)
@@ -186,6 +186,11 @@
 SYSCTL_INT(_vfs_zfs, OID_AUTO, mdcomp_disable, CTLFLAG_RDTUN,
     &zfs_mdcomp_disable, 0, "Disable metadata compression");
 
+#ifdef ZIO_USE_UMA
+extern kmem_cache_t	*zio_buf_cache[];
+extern kmem_cache_t	*zio_data_buf_cache[];
+#endif
+
 /*
  * Note that buffers can be in one of 6 states:
  *	ARC_anon	- anonymous (discussed below)
@@ -1893,8 +1898,6 @@
 	size_t			i;
 	kmem_cache_t		*prev_cache = NULL;
 	kmem_cache_t		*prev_data_cache = NULL;
-	extern kmem_cache_t	*zio_buf_cache[];
-	extern kmem_cache_t	*zio_data_buf_cache[];
 #endif
 
 #ifdef _KERNEL
Index: sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zio.c
===================================================================
--- sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zio.c	(revision 196083)
+++ sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zio.c	(working copy)
@@ -91,13 +91,6 @@
 #ifdef ZIO_USE_UMA
 	size_t c;
 #endif
-#if 0
-	vmem_t *data_alloc_arena = NULL;
-
-#ifdef _KERNEL
-	data_alloc_arena = zio_alloc_arena;
-#endif
-#endif
 	zio_cache = kmem_cache_create("zio_cache", sizeof (zio_t), 0,
 	    NULL, NULL, NULL, NULL, NULL, 0);
 
@@ -132,7 +125,7 @@
 
 			(void) sprintf(name, "zio_data_buf_%lu", (ulong_t)size);
 			zio_data_buf_cache[c] = kmem_cache_create(name, size,
-			    align, NULL, NULL, NULL, NULL, data_alloc_arena,
+			    align, NULL, NULL, NULL, NULL, NULL,
 			    KMC_NODEBUG);
 		}
 	}
@@ -422,7 +415,6 @@
 	ASSERT3U(size, <=, SPA_MAXBLOCKSIZE);
 	ASSERT(P2PHASE(size, SPA_MINBLOCKSIZE) == 0);
 	ASSERT(P2PHASE(offset, SPA_MINBLOCKSIZE) == 0);
-
 	ASSERT(!vd || spa_config_held(spa, SCL_STATE_ALL, RW_READER));
 	ASSERT(!bp || !(flags & ZIO_FLAG_CONFIG_WRITER));
 	ASSERT(vd || stage == ZIO_STAGE_OPEN);