3970 add tunable for maximum number of blocks freed in one txg
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Adam Leventhal <ahl@delphix.com>

*** 68,77 ****
--- 68,79 ----
  int zfs_resilver_min_time_ms = 3000; /* min millisecs to resilver per txg */
  boolean_t zfs_no_scrub_io = B_FALSE; /* set to disable scrub i/o */
  boolean_t zfs_no_scrub_prefetch = B_FALSE; /* set to disable srub prefetching */
  enum ddt_class zfs_scrub_ddt_class_max = DDT_CLASS_DUPLICATE;
  int dsl_scan_delay_completion = B_FALSE; /* set to delay scan completion */
+ /* max number of blocks to free in a single TXG */
+ uint64_t zfs_free_max_blocks = UINT64_MAX;
  
  #define DSL_SCAN_IS_SCRUB_RESILVER(scn) \
          ((scn)->scn_phys.scn_func == POOL_SCAN_SCRUB || \
          (scn)->scn_phys.scn_func == POOL_SCAN_RESILVER)
  
*** 1313,1322 ****
--- 1315,1327 ----
  static boolean_t
  dsl_scan_free_should_pause(dsl_scan_t *scn)
  {
          uint64_t elapsed_nanosecs;
  
+         if (scn->scn_visited_this_txg >= zfs_free_max_blocks)
+                 return (B_TRUE);
+ 
          elapsed_nanosecs = gethrtime() - scn->scn_sync_start_time;
          return (elapsed_nanosecs / NANOSEC > zfs_txg_timeout ||
              (NSEC2MSEC(elapsed_nanosecs) > zfs_free_min_time_ms &&
              txg_sync_waiting(scn->scn_dp)) ||
              spa_shutting_down(scn->scn_dp->dp_spa));