DLPX-62600 zfs sharenfs commands make slow progress on scalability sy…

…stem (openzfs#45)

cmd/zpool/zpool_main.c

@@ -1635,12 +1635,9 @@ zpool_export_one(zpool_handle_t *zhp, void *data)
 {
 	export_cbdata_t *cb = data;
 
-	verify(sharetab_lock() == 0);
 	if (zpool_disable_datasets(zhp, cb->force) != 0) {
-		verify(sharetab_unlock() == 0);
 		return (1);
 	}
-	verify(sharetab_unlock() == 0);
 
 	/* The history must be logged as part of the export */
 	log_history = B_FALSE;
@@ -1714,7 +1711,9 @@ zpool_do_export(int argc, char **argv)
 		usage(B_FALSE);
 	}
 
+	verify(sharetab_lock() == 0);
 	ret = for_each_pool(argc, argv, B_TRUE, NULL, zpool_export_one, &cb);
+	verify(sharetab_unlock() == 0);
 
 	return (ret);
 }

lib/libshare/libshare.c

@@ -91,6 +91,20 @@ sa_init(int init_service)
 
 	impl_handle->zfs_libhandle = libzfs_init();
 
+	/*
+	 * When libshare initializes the shares below, it needs to parse the
+	 * MNTTAB to determine if a filesystem is mounted by calling
+	 * zfs_is_mounted(). On systems with thousands of filesystems,
+	 * repeatedly parsing the MNTTAB can be very slow. As a performance
+	 * improvement, we cache the MNTTAB so that we only have to read it
+	 * once. Repeated calls to zfs_is_mounted will just lookup entries in
+	 * the AVL tree. We can only safely do this when we've locked the
+	 * sharetab to other zfs sharenfs operations from concurrently
+	 * updating the MNTTAB.
+	 */
+	if (sharetab_locked())
+		libzfs_mnttab_cache(impl_handle->zfs_libhandle, B_TRUE);
+
 	if (impl_handle->zfs_libhandle != NULL) {
 		libzfs_print_on_error(impl_handle->zfs_libhandle, B_TRUE);
 	}
@@ -217,6 +231,12 @@ sharetab_unlock(void)
 	return (retval);
 }
 
+boolean_t
+sharetab_locked(void)
+{
+	return(sharetab_fd != -1);
+}
+
 static void
 update_sharetab(sa_handle_impl_t impl_handle)
 {

lib/libspl/include/libshare.h

@@ -85,6 +85,7 @@ extern int sa_disable_share(sa_share_t, char *);
 
 extern int sharetab_lock(void);
 extern int sharetab_unlock(void);
+extern boolean_t sharetab_locked(void);
 
 /* protocol specific interfaces */
 extern int sa_parse_legacy_options(sa_group_t, char *, char *);