3756 want lz4 support for metadata compression

@@ -19,10 +19,11 @@
  * CDDL HEADER END
  */
 /*
  * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
  * Copyright (c) 2013 by Delphix. All rights reserved.
+ * Copyright (c) 2013 Martin Matuska. All rights reserved.
  */
 
 #include <sys/dmu.h>
 #include <sys/dmu_impl.h>
 #include <sys/dmu_tx.h>

@@ -40,10 +41,11 @@
 #include <sys/zfs_ioctl.h>
 #include <sys/zap.h>
 #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>
 #endif
 

@@ -1415,11 +1417,11 @@
         SET_BOOKMARK(&zb, ds->ds_object,
             db->db.db_object, db->db_level, db->db_blkid);
 
         DB_DNODE_ENTER(db);
         dn = DB_DNODE(db);
-        dmu_write_policy(os, dn, db->db_level, WP_DMU_SYNC, &zp);
+        dmu_write_policy(os, dn, db->db_level, WP_DMU_SYNC, &zp, txg);
         DB_DNODE_EXIT(db);
 
         /*
          * If we're frozen (running ziltest), we always need to generate a bp.
          */

@@ -1553,13 +1555,15 @@
         dnode_setdirty(dn, tx);
         dnode_rele(dn, FTAG);
 }
 
 int zfs_mdcomp_disable = 0;
+int zfs_mdcomp_lz4 = 0;
 
 void
-dmu_write_policy(objset_t *os, dnode_t *dn, int level, int wp, zio_prop_t *zp)
+dmu_write_policy(objset_t *os, dnode_t *dn, int level, int wp, zio_prop_t *zp,
+    uint64_t txg)
 {
         dmu_object_type_t type = dn ? dn->dn_type : DMU_OT_OBJSET;
         boolean_t ismd = (level > 0 || DMU_OT_IS_METADATA(type) ||
             (wp & WP_SPILL));
         enum zio_checksum checksum = os->os_checksum;

@@ -1580,12 +1584,30 @@
         if (ismd) {
                 /*
                  * XXX -- we should design a compression algorithm
                  * that specializes in arrays of bps.
                  */
-                compress = zfs_mdcomp_disable ? ZIO_COMPRESS_EMPTY :
-                    ZIO_COMPRESS_LZJB;
+                if (zfs_mdcomp_disable)
+                        compress = ZIO_COMPRESS_EMPTY;
+                else if (zfs_mdcomp_lz4 && os->os_spa != NULL) {
+                        zfeature_info_t *feat = &spa_feature_table
+                            [SPA_FEATURE_LZ4_COMPRESS];
+
+                        if (spa_feature_is_active(os->os_spa, feat))
+                                compress = ZIO_COMPRESS_LZ4;
+                        else if (spa_feature_is_enabled(os->os_spa, feat)) {
+                                dmu_tx_t *tx;
+
+                                tx = dmu_tx_create_assigned(
+                                    spa_get_dsl(os->os_spa), txg);
+                                spa_feature_incr(os->os_spa, feat, tx);
+                                dmu_tx_commit(tx);
+                                compress = ZIO_COMPRESS_LZ4;
+                        } else
+                                compress = ZIO_COMPRESS_LZJB;
+                } else
+                        compress = ZIO_COMPRESS_LZJB;
 
                 /*
                  * Metadata always gets checksummed.  If the data
                  * checksum is multi-bit correctable, and it's not a
                  * ZBT-style checksum, then it's suitable for metadata