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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.


  43         DTL_SCRUB,      /* unable to fully repair during scrub/resilver */
  44         DTL_OUTAGE,     /* temporarily missing (used to attempt detach) */
  45         DTL_TYPES
  46 } vdev_dtl_type_t;
  47 
  48 extern boolean_t zfs_nocacheflush;
  49 
  50 extern int vdev_open(vdev_t *);
  51 extern void vdev_open_children(vdev_t *);
  52 extern boolean_t vdev_uses_zvols(vdev_t *);
  53 extern int vdev_validate(vdev_t *, boolean_t);
  54 extern void vdev_close(vdev_t *);
  55 extern int vdev_create(vdev_t *, uint64_t txg, boolean_t isreplace);
  56 extern void vdev_reopen(vdev_t *);
  57 extern int vdev_validate_aux(vdev_t *vd);
  58 extern zio_t *vdev_probe(vdev_t *vd, zio_t *pio);
  59 
  60 extern boolean_t vdev_is_bootable(vdev_t *vd);
  61 extern vdev_t *vdev_lookup_top(spa_t *spa, uint64_t vdev);
  62 extern vdev_t *vdev_lookup_by_guid(vdev_t *vd, uint64_t guid);

  63 extern void vdev_dtl_dirty(vdev_t *vd, vdev_dtl_type_t d,
  64     uint64_t txg, uint64_t size);
  65 extern boolean_t vdev_dtl_contains(vdev_t *vd, vdev_dtl_type_t d,
  66     uint64_t txg, uint64_t size);
  67 extern boolean_t vdev_dtl_empty(vdev_t *vd, vdev_dtl_type_t d);
  68 extern void vdev_dtl_reassess(vdev_t *vd, uint64_t txg, uint64_t scrub_txg,
  69     int scrub_done);
  70 extern boolean_t vdev_dtl_required(vdev_t *vd);
  71 extern boolean_t vdev_resilver_needed(vdev_t *vd,
  72     uint64_t *minp, uint64_t *maxp);
  73 
  74 extern void vdev_hold(vdev_t *);
  75 extern void vdev_rele(vdev_t *);
  76 
  77 extern int vdev_metaslab_init(vdev_t *vd, uint64_t txg);
  78 extern void vdev_metaslab_fini(vdev_t *vd);
  79 extern void vdev_metaslab_set_size(vdev_t *);
  80 extern void vdev_expand(vdev_t *vd, uint64_t txg);
  81 extern void vdev_split(vdev_t *vd);
  82 extern void vdev_deadman(vdev_t *vd);




  43         DTL_SCRUB,      /* unable to fully repair during scrub/resilver */
  44         DTL_OUTAGE,     /* temporarily missing (used to attempt detach) */
  45         DTL_TYPES
  46 } vdev_dtl_type_t;
  47 
  48 extern boolean_t zfs_nocacheflush;
  49 
  50 extern int vdev_open(vdev_t *);
  51 extern void vdev_open_children(vdev_t *);
  52 extern boolean_t vdev_uses_zvols(vdev_t *);
  53 extern int vdev_validate(vdev_t *, boolean_t);
  54 extern void vdev_close(vdev_t *);
  55 extern int vdev_create(vdev_t *, uint64_t txg, boolean_t isreplace);
  56 extern void vdev_reopen(vdev_t *);
  57 extern int vdev_validate_aux(vdev_t *vd);
  58 extern zio_t *vdev_probe(vdev_t *vd, zio_t *pio);
  59 
  60 extern boolean_t vdev_is_bootable(vdev_t *vd);
  61 extern vdev_t *vdev_lookup_top(spa_t *spa, uint64_t vdev);
  62 extern vdev_t *vdev_lookup_by_guid(vdev_t *vd, uint64_t guid);
  63 extern int vdev_count_leaves(spa_t *spa);
  64 extern void vdev_dtl_dirty(vdev_t *vd, vdev_dtl_type_t d,
  65     uint64_t txg, uint64_t size);
  66 extern boolean_t vdev_dtl_contains(vdev_t *vd, vdev_dtl_type_t d,
  67     uint64_t txg, uint64_t size);
  68 extern boolean_t vdev_dtl_empty(vdev_t *vd, vdev_dtl_type_t d);
  69 extern void vdev_dtl_reassess(vdev_t *vd, uint64_t txg, uint64_t scrub_txg,
  70     int scrub_done);
  71 extern boolean_t vdev_dtl_required(vdev_t *vd);
  72 extern boolean_t vdev_resilver_needed(vdev_t *vd,
  73     uint64_t *minp, uint64_t *maxp);
  74 
  75 extern void vdev_hold(vdev_t *);
  76 extern void vdev_rele(vdev_t *);
  77 
  78 extern int vdev_metaslab_init(vdev_t *vd, uint64_t txg);
  79 extern void vdev_metaslab_fini(vdev_t *vd);
  80 extern void vdev_metaslab_set_size(vdev_t *);
  81 extern void vdev_expand(vdev_t *vd, uint64_t txg);
  82 extern void vdev_split(vdev_t *vd);
  83 extern void vdev_deadman(vdev_t *vd);