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.

*** 622,643 ****
          (void) zil_parse(zilog, zil_free_log_block,
              zil_free_log_record, tx, zilog->zl_header->zh_claim_txg);
  }
  
  int
! zil_claim(const char *osname, void *txarg)
  {
          dmu_tx_t *tx = txarg;
          uint64_t first_txg = dmu_tx_get_txg(tx);
          zilog_t *zilog;
          zil_header_t *zh;
          objset_t *os;
          int error;
  
!         error = dmu_objset_own(osname, DMU_OST_ANY, B_FALSE, FTAG, &os);
          if (error != 0) {
!                 cmn_err(CE_WARN, "can't open objset for %s", osname);
                  return (0);
          }
  
          zilog = dmu_objset_zil(os);
          zh = zil_header_in_syncing_context(zilog);
--- 622,645 ----
          (void) zil_parse(zilog, zil_free_log_block,
              zil_free_log_record, tx, zilog->zl_header->zh_claim_txg);
  }
  
  int
! zil_claim(dsl_pool_t *dp, dsl_dataset_t *ds, void *txarg)
  {
          dmu_tx_t *tx = txarg;
          uint64_t first_txg = dmu_tx_get_txg(tx);
          zilog_t *zilog;
          zil_header_t *zh;
          objset_t *os;
          int error;
  
!         error = dmu_objset_own_obj(dp, ds->ds_object,
!             DMU_OST_ANY, B_FALSE, FTAG, &os);
          if (error != 0) {
!                 cmn_err(CE_WARN, "can't open objset %llu, error %d",
!                     (unsigned long long)ds->ds_object, error);
                  return (0);
          }
  
          zilog = dmu_objset_zil(os);
          zh = zil_header_in_syncing_context(zilog);
*** 680,701 ****
   * Check the log by walking the log chain.
   * Checksum errors are ok as they indicate the end of the chain.
   * Any other error (no device or read failure) returns an error.
   */
  int
! zil_check_log_chain(const char *osname, void *tx)
  {
          zilog_t *zilog;
          objset_t *os;
          blkptr_t *bp;
          int error;
  
          ASSERT(tx == NULL);
  
!         error = dmu_objset_hold(osname, FTAG, &os);
          if (error != 0) {
!                 cmn_err(CE_WARN, "can't open objset for %s", osname);
                  return (0);
          }
  
          zilog = dmu_objset_zil(os);
          bp = (blkptr_t *)&zilog->zl_header->zh_log;
--- 682,704 ----
   * Check the log by walking the log chain.
   * Checksum errors are ok as they indicate the end of the chain.
   * Any other error (no device or read failure) returns an error.
   */
  int
! zil_check_log_chain(dsl_pool_t *dp, dsl_dataset_t *ds, void *tx)
  {
          zilog_t *zilog;
          objset_t *os;
          blkptr_t *bp;
          int error;
  
          ASSERT(tx == NULL);
  
!         error = dmu_objset_from_ds(ds, &os);
          if (error != 0) {
!                 cmn_err(CE_WARN, "can't open objset %llu, error %d",
!                     (unsigned long long)ds->ds_object, error);
                  return (0);
          }
  
          zilog = dmu_objset_zil(os);
          bp = (blkptr_t *)&zilog->zl_header->zh_log;
*** 714,728 ****
                  vd = vdev_lookup_top(os->os_spa, DVA_GET_VDEV(&bp->blk_dva[0]));
                  if (vd->vdev_islog && vdev_is_dead(vd))
                          valid = vdev_log_state_valid(vd);
                  spa_config_exit(os->os_spa, SCL_STATE, FTAG);
  
!                 if (!valid) {
!                         dmu_objset_rele(os, FTAG);
                          return (0);
                  }
-         }
  
          /*
           * Because tx == NULL, zil_claim_log_block() will not actually claim
           * any blocks, but just determine whether it is possible to do so.
           * In addition to checking the log chain, zil_claim_log_block()
--- 717,729 ----
                  vd = vdev_lookup_top(os->os_spa, DVA_GET_VDEV(&bp->blk_dva[0]));
                  if (vd->vdev_islog && vdev_is_dead(vd))
                          valid = vdev_log_state_valid(vd);
                  spa_config_exit(os->os_spa, SCL_STATE, FTAG);
  
!                 if (!valid)
                          return (0);
          }
  
          /*
           * Because tx == NULL, zil_claim_log_block() will not actually claim
           * any blocks, but just determine whether it is possible to do so.
           * In addition to checking the log chain, zil_claim_log_block()
*** 730,741 ****
           * which will update spa_max_claim_txg.  See spa_load() for details.
           */
          error = zil_parse(zilog, zil_claim_log_block, zil_claim_log_record, tx,
              zilog->zl_header->zh_claim_txg ? -1ULL : spa_first_txg(os->os_spa));
  
-         dmu_objset_rele(os, FTAG);
- 
          return ((error == ECKSUM || error == ENOENT) ? 0 : error);
  }
  
  static int
  zil_vdev_compare(const void *x1, const void *x2)
--- 731,740 ----