5269 zfs: zpool import slow
While importing a pool all objsets are enumerated twice, once to check
the zil log chains and once to claim them. On pools with many datasets
this process might take a substantial amount of time.
Speed up the process by parallelizing it utilizing a taskq. The number
of parallel tasks is limited to 4 times the number of leaf vdevs.

*** 1710,1726 ****
   */
  static boolean_t
  spa_check_logs(spa_t *spa)
  {
          boolean_t rv = B_FALSE;
  
          switch (spa->spa_log_state) {
          case SPA_LOG_MISSING:
                  /* need to recheck in case slog has been restored */
          case SPA_LOG_UNKNOWN:
!                 rv = (dmu_objset_find(spa->spa_name, zil_check_log_chain,
!                     NULL, DS_FIND_CHILDREN) != 0);
                  if (rv)
                          spa_set_log_state(spa, SPA_LOG_MISSING);
                  break;
          }
          return (rv);
--- 1710,1727 ----
   */
  static boolean_t
  spa_check_logs(spa_t *spa)
  {
          boolean_t rv = B_FALSE;
+         dsl_pool_t *dp = spa_get_dsl(spa);
  
          switch (spa->spa_log_state) {
          case SPA_LOG_MISSING:
                  /* need to recheck in case slog has been restored */
          case SPA_LOG_UNKNOWN:
!                 rv = (dmu_objset_find_dp(dp, dp->dp_root_dir_obj,
!                     zil_check_log_chain, NULL, DS_FIND_CHILDREN) != 0);
                  if (rv)
                          spa_set_log_state(spa, SPA_LOG_MISSING);
                  break;
          }
          return (rv);
*** 2090,2100 ****
          uint64_t children, config_cache_txg = spa->spa_config_txg;
          int orig_mode = spa->spa_mode;
          int parse;
          uint64_t obj;
          boolean_t missing_feat_write = B_FALSE;
- 
          /*
           * If this is an untrusted config, access the pool in read-only mode.
           * This prevents things like resilvering recently removed devices.
           */
          if (!mosconfig)
--- 2091,2100 ----
*** 2644,2653 ****
--- 2644,2654 ----
  
          if (spa_writeable(spa) && (state == SPA_LOAD_RECOVER ||
              spa->spa_load_max_txg == UINT64_MAX)) {
                  dmu_tx_t *tx;
                  int need_update = B_FALSE;
+                 dsl_pool_t *dp = spa_get_dsl(spa);
  
                  ASSERT(state != SPA_LOAD_TRYIMPORT);
  
                  /*
                   * Claim log blocks that haven't been committed yet.
*** 2656,2668 ****
                   * invoked from zil_claim_log_block()'s i/o done callback.
                   * Price of rollback is that we abandon the log.
                   */
                  spa->spa_claiming = B_TRUE;
  
!                 tx = dmu_tx_create_assigned(spa_get_dsl(spa),
!                     spa_first_txg(spa));
!                 (void) dmu_objset_find(spa_name(spa),
                      zil_claim, tx, DS_FIND_CHILDREN);
                  dmu_tx_commit(tx);
  
                  spa->spa_claiming = B_FALSE;
  
--- 2657,2668 ----
                   * invoked from zil_claim_log_block()'s i/o done callback.
                   * Price of rollback is that we abandon the log.
                   */
                  spa->spa_claiming = B_TRUE;
  
!                 tx = dmu_tx_create_assigned(dp, spa_first_txg(spa));
!                 (void) dmu_objset_find_dp(dp, dp->dp_root_dir_obj,
                      zil_claim, tx, DS_FIND_CHILDREN);
                  dmu_tx_commit(tx);
  
                  spa->spa_claiming = B_FALSE;