Allow empty ds_props_obj to be destroyed

Currently, 'zfs list' and 'zfs get' commands can be slow when
working with snapshots that have a ds_props_obj. This is
because the code that discovers all of the properties for these
snapshots needs to read this object for each snapshot, which
almost always ends up causing an extra random synchronous read
for each snapshot. This performance penalty exists even if the
properties on that snapshot have been unset because the object
is normally only freed when the snapshot is freed, even though
it is only created when it is needed.

This patch allows the user to regain 'zfs list' performance on
these snapshots by destroying the ds_props_obj when it no longer
has any entries left. In practice on a production machine, this
optimization seems to make 'zfs list' about 55% faster.

Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Paul Zuchowski <[email protected]>
Signed-off-by: Tom Caputi <[email protected]>
Closes #9704

module/zfs/dsl_prop.c

@@ -649,7 +649,7 @@ dsl_prop_set_sync_impl(dsl_dataset_t *ds, const char *propname,
     dmu_tx_t *tx)
 {
 	objset_t *mos = ds->ds_dir->dd_pool->dp_meta_objset;
-	uint64_t zapobj, intval, dummy;
+	uint64_t zapobj, intval, dummy, count;
 	int isint;
 	char valbuf[32];
 	const char *valstr = NULL;
@@ -663,7 +663,8 @@ dsl_prop_set_sync_impl(dsl_dataset_t *ds, const char *propname,
 
 	if (ds->ds_is_snapshot) {
 		ASSERT(version >= SPA_VERSION_SNAP_PROPS);
-		if (dsl_dataset_phys(ds)->ds_props_obj == 0) {
+		if (dsl_dataset_phys(ds)->ds_props_obj == 0 &&
+		    (source & ZPROP_SRC_NONE) == 0) {
 			dmu_buf_will_dirty(ds->ds_dbuf, tx);
 			dsl_dataset_phys(ds)->ds_props_obj =
 			    zap_create(mos,
@@ -674,6 +675,10 @@ dsl_prop_set_sync_impl(dsl_dataset_t *ds, const char *propname,
 		zapobj = dsl_dir_phys(ds->ds_dir)->dd_props_zapobj;
 	}
 
+	/* If we are removing objects from a non-existent ZAP just return */
+	if (zapobj == 0)
+		return;
+
 	if (version < SPA_VERSION_RECVD_PROPS) {
 		if (source & ZPROP_SRC_NONE)
 			source = ZPROP_SRC_NONE;
@@ -755,6 +760,18 @@ dsl_prop_set_sync_impl(dsl_dataset_t *ds, const char *propname,
 	kmem_strfree(inheritstr);
 	kmem_strfree(recvdstr);
 
+	/*
+	 * If we are left with an empty snap zap we can destroy it.
+	 * This will prevent unnecessary calls to zap_lookup() in
+	 * the "zfs list" and "zfs get" code paths.
+	 */
+	if (ds->ds_is_snapshot &&
+	    zap_count(mos, zapobj, &count) == 0 && count == 0) {
+		dmu_buf_will_dirty(ds->ds_dbuf, tx);
+		dsl_dataset_phys(ds)->ds_props_obj = 0;
+		zap_destroy(mos, zapobj, tx);
+	}
+
 	if (isint) {
 		VERIFY0(dsl_prop_get_int_ds(ds, propname, &intval));