3740 Poor ZFS send / receive performance due to snapshot hold / release processing
Submitted by: Steven Hartland <steven.hartland@multiplay.co.uk>

*** 344,358 ****
   * In this case, when the cleanup_fd is closed (including on process
   * termination), the holds will be released.  If the system is shut down
   * uncleanly, the holds will be released when the pool is next opened
   * or imported.
   *
!  * The return value will be 0 if all holds were created. Otherwise the return
!  * value will be the errno of a (unspecified) hold that failed, no holds will
!  * be created, and the errlist will have an entry for each hold that
!  * failed (name = snapshot).  The value in the errlist will be the error
!  * code (int32).
   */
  int
  lzc_hold(nvlist_t *holds, int cleanup_fd, nvlist_t **errlist)
  {
          char pool[MAXNAMELEN];
--- 344,369 ----
   * In this case, when the cleanup_fd is closed (including on process
   * termination), the holds will be released.  If the system is shut down
   * uncleanly, the holds will be released when the pool is next opened
   * or imported.
   *
!  * Holds for snapshots which don't exist will be skipped and have an entry
!  * added to errlist, but will not cause an overall failure, except in the
!  * case that all holds where skipped.
!  *
!  * The return value will be 0 if the nvl holds was empty or all holds, for
!  * snapshots that existed, were succesfully created and at least one hold
!  * was created.
!  *
!  * If none of the snapshots for the requested holds existed ENOENT will be
!  * returned.
!  *
!  * Otherwise the return value will be the errno of a (unspecified) hold that
!  * failed, no holds will be created.
!  *
!  * In all cases the errlist will have an entry for each hold that failed
!  * (name = snapshot), with its value being the error code (int32).
   */
  int
  lzc_hold(nvlist_t *holds, int cleanup_fd, nvlist_t **errlist)
  {
          char pool[MAXNAMELEN];
*** 385,399 ****
   *
   * The keys in the nvlist are snapshot names.
   * The snapshots must all be in the same pool.
   * The value is a nvlist whose keys are the holds to remove.
   *
!  * The return value will be 0 if all holds were removed.
!  * Otherwise the return value will be the errno of a (unspecified) release
!  * that failed, no holds will be released, and the errlist will have an
!  * entry for each snapshot that has failed releases (name = snapshot).
!  * The value in the errlist will be the error code (int32) of a failed release.
   */
  int
  lzc_release(nvlist_t *holds, nvlist_t **errlist)
  {
          char pool[MAXNAMELEN];
--- 396,418 ----
   *
   * The keys in the nvlist are snapshot names.
   * The snapshots must all be in the same pool.
   * The value is a nvlist whose keys are the holds to remove.
   *
!  * Holds which failed to release because they didn't exist will have an entry
!  * added to errlist, but will not cause an overall failure.
!  *
!  * The return value will be 0 if the nvl holds was empty or all holds, that
!  * existed, were succesfully removed and at least one hold was removed.
!  *
!  * If none of the holds specified existed ENOENT will be returned.
!  *
!  * Otherwise the return value will be the errno of a (unspecified) hold that
!  * failed to release and no holds will be released.
!  *
!  * In all cases the errlist will have an entry for each hold that failed to
!  * to release.
   */
  int
  lzc_release(nvlist_t *holds, nvlist_t **errlist)
  {
          char pool[MAXNAMELEN];