Illumos 4924 - LZ4 Compression for metadata

Reviewed by Matthew Ahrens <mahrens@delphix.com>
Reviewed by Saso Kiselkov <skiselkov.ml@gmail.com>
Approved by: Christopher Siden <christopher.siden@delphix.com>

References:
  https://github.com/illumos/illumos-gate/commit/b8289d2
  https://www.illumos.org/issues/3756

Porting notes:

The static function zfs_prop_activate_feature() was removed because
this change removes the only caller.  The function was not removed
from Illumos but instead left as dead code.  However, to keep gcc
happy it was removed from Linux and may be easily restored if needed.

Ported by: DHE <git@dehacked.net>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1540

module/zfs/dmu.c

@@ -22,6 +22,7 @@
  * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
  * Copyright (c) 2011, 2014 by Delphix. All rights reserved.
  * Copyright (c) 2013 by Saso Kiselkov. All rights reserved.
+ * Copyright (c) 2014, Nexenta Systems, Inc. All rights reserved.
  */
 
 #include <sys/dmu.h>
@@ -43,6 +44,7 @@
 #include <sys/zio_checksum.h>
 #include <sys/zio_compress.h>
 #include <sys/sa.h>
+#include <sys/zfeature.h>
 #ifdef _KERNEL
 #include <sys/vmsystm.h>
 #include <sys/zfs_znode.h>
@@ -1765,8 +1767,16 @@ dmu_write_policy(objset_t *os, dnode_t *dn, int level, int wp, zio_prop_t *zp)
 		 * XXX -- we should design a compression algorithm
 		 * that specializes in arrays of bps.
 		 */
-		compress = zfs_mdcomp_disable ? ZIO_COMPRESS_EMPTY :
-		    ZIO_COMPRESS_LZJB;
+		boolean_t lz4_ac = spa_feature_is_active(os->os_spa,
+		    SPA_FEATURE_LZ4_COMPRESS);
+
+		if (zfs_mdcomp_disable) {
+			compress = ZIO_COMPRESS_EMPTY;
+		} else if (lz4_ac) {
+			compress = ZIO_COMPRESS_LZ4;
+		} else {
+			compress = ZIO_COMPRESS_LZJB;
+		}
 
 		/*
 		 * Metadata always gets checksummed.  If the data