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Optimize creation and removal of temporary "user holds" placed on
snapshots by a zfs send, by ensuring all the required holds and
releases are done in a single dsl_sync_task.
Creation now collates the required holds during a dry run and
then uses a single lzc_hold call via zfs_hold_apply instead of
processing each snapshot in turn.
Defered (on exit) cleanup by the kernel is also now done in
dsl_sync_task by reusing dsl_dataset_user_release.
On a test with 11 volumes in a tree each with 8 snapshots on a
single HDD zpool this reduces the time required to perform a full
send from 20 seconds to under 0.8 seconds.
For reference eliminating the hold entirely reduces this 0.15
seconds.
While I'm here:-
* Remove some unused structures
* Fix nvlist_t leak in zfs_release_one

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          --- old/usr/src/lib/libzfs/common/libzfs.h
          +++ new/usr/src/lib/libzfs/common/libzfs.h
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 588  588  } sendflags_t;
 589  589  
 590  590  typedef boolean_t (snapfilter_cb_t)(zfs_handle_t *, void *);
 591  591  
 592  592  extern int zfs_send(zfs_handle_t *, const char *, const char *,
 593  593      sendflags_t *, int, snapfilter_cb_t, void *, nvlist_t **);
 594  594  
 595  595  extern int zfs_promote(zfs_handle_t *);
 596  596  extern int zfs_hold(zfs_handle_t *, const char *, const char *,
 597  597      boolean_t, boolean_t, int);
      598 +extern int zfs_hold_add(zfs_handle_t *, const char *, const char *,
      599 +    boolean_t, nvlist_t *);
      600 +extern int zfs_hold_apply(zfs_handle_t *, boolean_t, int, nvlist_t *);
 598  601  extern int zfs_release(zfs_handle_t *, const char *, const char *, boolean_t);
 599  602  extern int zfs_get_holds(zfs_handle_t *, nvlist_t **);
 600  603  extern uint64_t zvol_volsize_to_reservation(uint64_t, nvlist_t *);
 601  604  
 602  605  typedef int (*zfs_userspace_cb_t)(void *arg, const char *domain,
 603  606      uid_t rid, uint64_t space);
 604  607  
 605  608  extern int zfs_userspace(zfs_handle_t *, zfs_userquota_prop_t,
 606  609      zfs_userspace_cb_t, void *);
 607  610  
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