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

@@ -252,12 +252,15 @@
  *
  * If 'defer' is set, and a snapshot has user holds or clones, it will be
  * marked for deferred destruction, and will be destroyed when the last hold
  * or clone is removed/destroyed.
  *
+ * The return value will be ENOENT if none of the snapshots existed.
+ *
  * The return value will be 0 if all snapshots were destroyed (or marked for
- * later destruction if 'defer' is set) or didn't exist to begin with.
+ * later destruction if 'defer' is set) or didn't exist to begin with and
+ * at least one snapshot was destroyed.
  *
  * Otherwise the return value will be the errno of a (unspecified) snapshot
  * that failed, no snapshots will be destroyed, and the errlist will have an
  * entry for each snapshot that failed.  The value in the errlist will be
  * the (int32) error code.

@@ -284,11 +287,10 @@
 
         error = lzc_ioctl(ZFS_IOC_DESTROY_SNAPS, pool, args, errlist);
         nvlist_free(args);
 
         return (error);
-
 }
 
 int
 lzc_snaprange_space(const char *firstsnap, const char *lastsnap,
     uint64_t *usedp)

@@ -344,15 +346,26 @@
  * 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 ENOENT if none of the snapshots for the requested
+ * holds existed.
+ *
+ * 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.
+ *
+ * Otherwise the return value will be the errno of a (unspecified) hold that
+ * failed and 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 +398,24 @@
  *
  * 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, except in the
+ * case that all releases where skipped.
+ *
+ * The return value will be ENOENT if none of the specified holds existed.
+ *
+ * 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.
+ *
+ * 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];