Add snapshots_changed as property

Make dd_snap_cmtime property persistent across mount and unmount
operations by storing in ZAP and restore the value from ZAP on hold
into dd_snap_cmtime instead of updating it.

Expose dd_snap_cmtime as 'snapshots_changed' property that provides a
mechanism to quickly determine whether snapshot list for dataset has
changed without having to mount a dataset or iterate the snapshot list.

It specifies the time at which a snapshot for a dataset was last
created or deleted. This allows us to be more efficient how often we
query snapshots.

Reviewed-by: Ryan Moeller <[email protected]>
Reviewed-by: Alexander Motin <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Umer Saleem <[email protected]>
Closes #13635 

4 files changed, 27 insertions, 5 deletions

diff --git a/module/zcommon/zfs_prop.c b/module/zcommon/zfs_prop.c
index 0b0fc9015..2a58088a7 100644
--- a/module/zcommon/zfs_prop.c
+++ b/module/zcommon/zfs_prop.c
@@ -734,6 +734,11 @@ zfs_prop_init(void)
 	    NULL, PROP_READONLY, ZFS_TYPE_DATASET | ZFS_TYPE_BOOKMARK,
 	    "<date>", "CREATION", B_FALSE, B_TRUE, B_TRUE, NULL, sfeatures);
 
+	zprop_register_impl(ZFS_PROP_SNAPSHOTS_CHANGED, "snapshots_changed",
+	    PROP_TYPE_NUMBER, 0, NULL, PROP_READONLY, ZFS_TYPE_FILESYSTEM |
+	    ZFS_TYPE_VOLUME, "<date>", "SNAPSHOTS_CHANGED", B_FALSE, B_TRUE,
+	    B_TRUE, NULL, sfeatures);
+
 	zfs_mod_list_supported_free(sfeatures);
 }
 
diff --git a/module/zfs/dsl_dataset.c b/module/zfs/dsl_dataset.c
index 5f1235583..12fcecf0d 100644
--- a/module/zfs/dsl_dataset.c
+++ b/module/zfs/dsl_dataset.c
@@ -534,7 +534,7 @@ dsl_dataset_snap_remove(dsl_dataset_t *ds, const char *name, dmu_tx_t *tx,
 	matchtype_t mt = 0;
 	int err;
 
-	dsl_dir_snap_cmtime_update(ds->ds_dir);
+	dsl_dir_snap_cmtime_update(ds->ds_dir, tx);
 
 	if (dsl_dataset_phys(ds)->ds_flags & DS_FLAG_CI_DATASET)
 		mt = MT_NORMALIZE;
@@ -1865,7 +1865,7 @@ dsl_dataset_snapshot_sync_impl(dsl_dataset_t *ds, const char *snapname,
 
 	dsl_scan_ds_snapshotted(ds, tx);
 
-	dsl_dir_snap_cmtime_update(ds->ds_dir);
+	dsl_dir_snap_cmtime_update(ds->ds_dir, tx);
 
 	spa_history_log_internal_ds(ds->ds_prev, "snapshot", tx, " ");
 }
@@ -2809,6 +2809,8 @@ dsl_dataset_stats(dsl_dataset_t *ds, nvlist_t *nv)
 	    dsl_get_userrefs(ds));
 	dsl_prop_nvlist_add_uint64(nv, ZFS_PROP_DEFER_DESTROY,
 	    dsl_get_defer_destroy(ds));
+	dsl_prop_nvlist_add_uint64(nv, ZFS_PROP_SNAPSHOTS_CHANGED,
+	    dsl_dir_snap_cmtime(ds->ds_dir).tv_sec);
 	dsl_dataset_crypt_stats(ds, nv);
 
 	if (dsl_dataset_phys(ds)->ds_prev_snap_obj != 0) {
diff --git a/module/zfs/dsl_dir.c b/module/zfs/dsl_dir.c
index cc870d1bc..5a64e399c 100644
--- a/module/zfs/dsl_dir.c
+++ b/module/zfs/dsl_dir.c
@@ -207,8 +207,6 @@ dsl_dir_hold_obj(dsl_pool_t *dp, uint64_t ddobj,
 			}
 		}
 
-		dsl_dir_snap_cmtime_update(dd);
-
 		if (dsl_dir_phys(dd)->dd_parent_obj) {
 			err = dsl_dir_hold_obj(dp,
 			    dsl_dir_phys(dd)->dd_parent_obj, NULL, dd,
@@ -270,6 +268,14 @@ dsl_dir_hold_obj(dsl_pool_t *dp, uint64_t ddobj,
 			}
 		}
 
+		inode_timespec_t t = {0};
+		zap_lookup(dd->dd_pool->dp_meta_objset,
+		    dsl_dir_phys(dd)->dd_props_zapobj,
+		    zfs_prop_to_name(ZFS_PROP_SNAPSHOTS_CHANGED),
+		    sizeof (uint64_t),
+		    sizeof (inode_timespec_t) / sizeof (uint64_t), &t);
+		dd->dd_snap_cmtime = t;
+
 		dmu_buf_init_user(&dd->dd_dbu, NULL, dsl_dir_evict_async,
 		    &dd->dd_dbuf);
 		winner = dmu_buf_set_user_ie(dbuf, &dd->dd_dbu);
@@ -2243,13 +2249,18 @@ dsl_dir_snap_cmtime(dsl_dir_t *dd)
 }
 
 void
-dsl_dir_snap_cmtime_update(dsl_dir_t *dd)
+dsl_dir_snap_cmtime_update(dsl_dir_t *dd, dmu_tx_t *tx)
 {
 	inode_timespec_t t;
+	objset_t *mos = dd->dd_pool->dp_meta_objset;
+	uint64_t zapobj = dsl_dir_phys(dd)->dd_props_zapobj;
+	const char *prop_name = zfs_prop_to_name(ZFS_PROP_SNAPSHOTS_CHANGED);
 
 	gethrestime(&t);
 	mutex_enter(&dd->dd_lock);
 	dd->dd_snap_cmtime = t;
+	VERIFY0(zap_update(mos, zapobj, prop_name, sizeof (uint64_t),
+	    sizeof (inode_timespec_t) / sizeof (uint64_t), &t, tx));
 	mutex_exit(&dd->dd_lock);
 }
 
diff --git a/module/zfs/zcp_get.c b/module/zfs/zcp_get.c
index af495ce85..0a0466d46 100644
--- a/module/zfs/zcp_get.c
+++ b/module/zfs/zcp_get.c
@@ -403,6 +403,10 @@ get_special_prop(lua_State *state, dsl_dataset_t *ds, const char *dsname,
 		break;
 	}
 
+	case ZFS_PROP_SNAPSHOTS_CHANGED:
+		numval = dsl_dir_snap_cmtime(ds->ds_dir).tv_sec;
+		break;
+
 	default:
 		/* Did not match these props, check in the dsl_dir */
 		error = get_dsl_dir_prop(ds, zfs_prop, &numval);