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

          --- old/usr/src/uts/common/fs/zfs/sys/dsl_userhold.h
          +++ new/usr/src/uts/common/fs/zfs/sys/dsl_userhold.h

   1    1  
   2    2  /*
   3    3   * CDDL HEADER START
   4    4   *
   5    5   * The contents of this file are subject to the terms of the
   6    6   * Common Development and Distribution License (the "License").
   7    7   * You may not use this file except in compliance with the License.
   8    8   *
   9    9   * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
  10   10   * or http://www.opensolaris.org/os/licensing.
  11   11   * See the License for the specific language governing permissions
  12   12   * and limitations under the License.
  13   13   *
  14   14   * When distributing Covered Code, include this CDDL HEADER in each
  15   15   * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
  16   16   * If applicable, add the following below this CDDL HEADER, with the
  17   17   * fields enclosed by brackets "[]" replaced with your own identifying
  18   18   * information: Portions Copyright [yyyy] [name of copyright owner]
  19   19   *
  20   20   * CDDL HEADER END
  21   21   */
  22   22  /*
  23   23   * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
  24   24   * Copyright (c) 2012 by Delphix. All rights reserved.
  25   25   * Copyright (c) 2012, Joyent, Inc. All rights reserved.
  26   26   */
  27   27  
  28   28  #ifndef _SYS_DSL_USERHOLD_H
  29   29  #define _SYS_DSL_USERHOLD_H
  30   30  
  31   31  #include <sys/nvpair.h>
  32   32  #include <sys/types.h>
  33   33  
  34   34  #ifdef  __cplusplus
  35   35  extern "C" {

  36   36  #endif
  37   37  
  38   38  struct dsl_pool;
  39   39  struct dsl_dataset;
  40   40  struct dmu_tx;
  41   41  
  42   42  int dsl_dataset_user_hold(nvlist_t *holds, minor_t cleanup_minor,
  43   43      nvlist_t *errlist);
  44   44  int dsl_dataset_user_release(nvlist_t *holds, nvlist_t *errlist);
  45   45  int dsl_dataset_get_holds(const char *dsname, nvlist_t *nvl);
  46      -void dsl_dataset_user_release_tmp(struct dsl_pool *dp, uint64_t dsobj,
  47      -    const char *htag);
  48   46  int dsl_dataset_user_hold_check_one(struct dsl_dataset *ds, const char *htag,
  49   47      boolean_t temphold, struct dmu_tx *tx);
  50   48  void dsl_dataset_user_hold_sync_one(struct dsl_dataset *ds, const char *htag,
  51   49      minor_t minor, uint64_t now, struct dmu_tx *tx);
  52   50  
  53   51  #ifdef  __cplusplus
  54   52  }
  55   53  #endif
  56   54  
  57   55  #endif /* _SYS_DSL_USERHOLD_H */
    

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