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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.


 120         sa_os_t *os_sa;
 121 };
 122 
 123 #define DMU_META_OBJSET         0
 124 #define DMU_META_DNODE_OBJECT   0
 125 #define DMU_OBJECT_IS_SPECIAL(obj) ((int64_t)(obj) <= 0)
 126 #define DMU_META_DNODE(os)      ((os)->os_meta_dnode.dnh_dnode)
 127 #define DMU_USERUSED_DNODE(os)  ((os)->os_userused_dnode.dnh_dnode)
 128 #define DMU_GROUPUSED_DNODE(os) ((os)->os_groupused_dnode.dnh_dnode)
 129 
 130 #define DMU_OS_IS_L2CACHEABLE(os)                               \
 131         ((os)->os_secondary_cache == ZFS_CACHE_ALL ||                \
 132         (os)->os_secondary_cache == ZFS_CACHE_METADATA)
 133 
 134 #define DMU_OS_IS_L2COMPRESSIBLE(os)    (zfs_mdcomp_disable == B_FALSE)
 135 
 136 /* called from zpl */
 137 int dmu_objset_hold(const char *name, void *tag, objset_t **osp);
 138 int dmu_objset_own(const char *name, dmu_objset_type_t type,
 139     boolean_t readonly, void *tag, objset_t **osp);


 140 void dmu_objset_refresh_ownership(objset_t *os, void *tag);
 141 void dmu_objset_rele(objset_t *os, void *tag);
 142 void dmu_objset_disown(objset_t *os, void *tag);
 143 int dmu_objset_from_ds(struct dsl_dataset *ds, objset_t **osp);
 144 
 145 void dmu_objset_stats(objset_t *os, nvlist_t *nv);
 146 void dmu_objset_fast_stat(objset_t *os, dmu_objset_stats_t *stat);
 147 void dmu_objset_space(objset_t *os, uint64_t *refdbytesp, uint64_t *availbytesp,
 148     uint64_t *usedobjsp, uint64_t *availobjsp);
 149 uint64_t dmu_objset_fsid_guid(objset_t *os);
 150 int dmu_objset_find_dp(struct dsl_pool *dp, uint64_t ddobj,
 151     int func(struct dsl_pool *, struct dsl_dataset *, void *),
 152     void *arg, int flags);
 153 int dmu_objset_prefetch(const char *name, void *arg);
 154 void dmu_objset_evict_dbufs(objset_t *os);
 155 timestruc_t dmu_objset_snap_cmtime(objset_t *os);
 156 
 157 /* called from dsl */
 158 void dmu_objset_sync(objset_t *os, zio_t *zio, dmu_tx_t *tx);
 159 boolean_t dmu_objset_is_dirty(objset_t *os, uint64_t txg);



 120         sa_os_t *os_sa;
 121 };
 122 
 123 #define DMU_META_OBJSET         0
 124 #define DMU_META_DNODE_OBJECT   0
 125 #define DMU_OBJECT_IS_SPECIAL(obj) ((int64_t)(obj) <= 0)
 126 #define DMU_META_DNODE(os)      ((os)->os_meta_dnode.dnh_dnode)
 127 #define DMU_USERUSED_DNODE(os)  ((os)->os_userused_dnode.dnh_dnode)
 128 #define DMU_GROUPUSED_DNODE(os) ((os)->os_groupused_dnode.dnh_dnode)
 129 
 130 #define DMU_OS_IS_L2CACHEABLE(os)                               \
 131         ((os)->os_secondary_cache == ZFS_CACHE_ALL ||                \
 132         (os)->os_secondary_cache == ZFS_CACHE_METADATA)
 133 
 134 #define DMU_OS_IS_L2COMPRESSIBLE(os)    (zfs_mdcomp_disable == B_FALSE)
 135 
 136 /* called from zpl */
 137 int dmu_objset_hold(const char *name, void *tag, objset_t **osp);
 138 int dmu_objset_own(const char *name, dmu_objset_type_t type,
 139     boolean_t readonly, void *tag, objset_t **osp);
 140 int dmu_objset_own_obj(dsl_pool_t *dp, uint64_t obj, dmu_objset_type_t type,
 141     boolean_t readonly, void *tag, objset_t **osp);
 142 void dmu_objset_refresh_ownership(objset_t *os, void *tag);
 143 void dmu_objset_rele(objset_t *os, void *tag);
 144 void dmu_objset_disown(objset_t *os, void *tag);
 145 int dmu_objset_from_ds(struct dsl_dataset *ds, objset_t **osp);
 146 
 147 void dmu_objset_stats(objset_t *os, nvlist_t *nv);
 148 void dmu_objset_fast_stat(objset_t *os, dmu_objset_stats_t *stat);
 149 void dmu_objset_space(objset_t *os, uint64_t *refdbytesp, uint64_t *availbytesp,
 150     uint64_t *usedobjsp, uint64_t *availobjsp);
 151 uint64_t dmu_objset_fsid_guid(objset_t *os);
 152 int dmu_objset_find_dp(struct dsl_pool *dp, uint64_t ddobj,
 153     int func(struct dsl_pool *, struct dsl_dataset *, void *),
 154     void *arg, int flags);
 155 int dmu_objset_prefetch(const char *name, void *arg);
 156 void dmu_objset_evict_dbufs(objset_t *os);
 157 timestruc_t dmu_objset_snap_cmtime(objset_t *os);
 158 
 159 /* called from dsl */
 160 void dmu_objset_sync(objset_t *os, zio_t *zio, dmu_tx_t *tx);
 161 boolean_t dmu_objset_is_dirty(objset_t *os, uint64_t txg);