3756 want lz4 support for metadata compression

*** 19,28 ****
--- 19,29 ----
   * 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>
*** 40,49 ****
--- 41,51 ----
  #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
  
*** 1415,1425 ****
          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);
          DB_DNODE_EXIT(db);
  
          /*
           * If we're frozen (running ziltest), we always need to generate a bp.
           */
--- 1417,1427 ----
          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, txg);
          DB_DNODE_EXIT(db);
  
          /*
           * If we're frozen (running ziltest), we always need to generate a bp.
           */
*** 1553,1565 ****
          dnode_setdirty(dn, tx);
          dnode_rele(dn, FTAG);
  }
  
  int zfs_mdcomp_disable = 0;
  
  void
! dmu_write_policy(objset_t *os, dnode_t *dn, int level, int wp, zio_prop_t *zp)
  {
          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;
--- 1555,1569 ----
          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,
!     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;
*** 1580,1591 ****
          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;
  
                  /*
                   * 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
--- 1584,1613 ----
          if (ismd) {
                  /*
                   * XXX -- we should design a compression algorithm
                   * that specializes in arrays of bps.
                   */
!                 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