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

@@ -344,15 +344,26 @@
  * 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).
+ * 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];

@@ -385,15 +396,23 @@
  *
  * 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.
+ * 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];