add no-upgrade features

Signed-off-by: Paul Dagnelie <[email protected]>

Author: Paul Dagnelie

include/zfeature_common.h

@@ -103,7 +103,14 @@ typedef enum zfeature_flags {
 	/* Activate this feature at the same time it is enabled. */
 	ZFEATURE_FLAG_ACTIVATE_ON_ENABLE =	(1 << 2),
 	/* Each dataset has a field set if it has ever used this feature. */
-	ZFEATURE_FLAG_PER_DATASET =		(1 << 3)
+	ZFEATURE_FLAG_PER_DATASET =		(1 << 3),
+	/*
+	 * This feature isn't enabled by zpool upgrade; it must be explicitly
+	 * listed to be enabled. This also applies to compatibility lists. This
+	 * flag can be removed from a given feature once support is sufficiently
+	 * widespread.
+	 */
+	ZFEATURE_FLAG_NO_UPGRADE = 		(1 << 4)
 } zfeature_flags_t;
 
 typedef enum zfeature_type {

lib/libzfs/libzfs_pool.c

@@ -5108,9 +5108,13 @@ zpool_load_compat(const char *compat, boolean_t *features, char *report,
 	/* special cases (unset), "" and "off" => enable all features */
 	if (compat == NULL || compat[0] == '\0' ||
 	    strcmp(compat, ZPOOL_COMPAT_OFF) == 0) {
-		if (features != NULL)
-			for (uint_t i = 0; i < SPA_FEATURES; i++)
-				features[i] = B_TRUE;
+		if (features != NULL) {
+			for (uint_t i = 0; i < SPA_FEATURES; i++) {
+				if (!(spa_feature_table[i].fi_flags &
+				    ZFEATURE_FLAG_NO_UPGRADE))
+					features[i] = B_TRUE;
+			}
+		}
 		if (report != NULL)
 			strlcpy(report, gettext("all features enabled"), rlen);
 		return (ZPOOL_COMPATIBILITY_OK);

man/man4/zfs.4

@@ -2499,20 +2499,6 @@ the xattr so as to not accumulate duplicates.
 .
 .It Sy zio_requeue_io_start_cut_in_line Ns = Ns Sy 0 Ns | Ns 1 Pq int
 Prioritize requeued I/O.
-.It Sy zio_dynamic_gang_headers_enable Ns = Ns Sy 0 Ns | Ns 1 Pq int
-Enable dynamically sized gang headers.
-.Pp
-When set to 0 (the default), the dynamic_gang_header feature will never be
-activated, even if it is enabled.
-All gang headers will store at most 3 children, regardless of the header's
-allocated size on disk.
-When set to 1, the dynamic_gang_header feature will be activated once a
-gang allocation is larger than 512 bytes.
-This will cause the gang header to store more more gang children on pools
-with larger ashifts.
-Enabling this flag and the feature will reduce multi-level gang trees,
-but can result in more IOs for individual gang blocks.
-.
 .It Sy zio_taskq_batch_pct Ns = Ns Sy 80 Ns % Pq uint
 Percentage of online CPUs which will run a worker thread for I/O.
 These workers are responsible for I/O work such as compression, encryption,

man/man7/zpool-features.7

@@ -492,6 +492,18 @@ vdev type, or when adding a new
 .Sy draid
 vdev to an existing pool.
 .
+.feature com.klarasystems dynamic_gang_header no
+This feature enables larger gang headers based on the sector size of the pool.
+When enabled, gang headers will use the entire space allocated for them, instead
+of always restricting themselves to 512 bytes.
+This can reduce the need for nested gang trees in extreme fragmentation scenarios.
+.Pp
+This feature becomes active when a gang header is written that is larger than
+512 bytes.
+This feature is not enabled by
+.Xr zpool-upgrade 8 .
+Instead, it must be manually enabled, or be part of a compatibility file.
+.
 .feature org.illumos edonr no extensible_dataset
 This feature enables the use of the Edon-R hash algorithm for checksum,
 including for nopwrite

module/zcommon/zfeature_common.c

@@ -788,8 +788,8 @@ zpool_feature_init(void)
 	zfeature_register(SPA_FEATURE_DYNAMIC_GANG_HEADER,
 	    "com.klarasystems:dynamic_gang_header", "dynamic_gang_header",
 	    "Support for dynamically sized gang headers",
-	    ZFEATURE_FLAG_MOS, ZFEATURE_TYPE_BOOLEAN, NULL,
-	    sfeatures);
+	    ZFEATURE_FLAG_MOS | ZFEATURE_FLAG_NO_UPGRADE,
+	    ZFEATURE_TYPE_BOOLEAN, NULL, sfeatures);
 
 	zfs_mod_list_supported_free(sfeatures);
 }

module/zfs/zio.c

@@ -139,8 +139,6 @@ static uint_t zfs_sync_pass_rewrite = 2;
 int zio_exclude_metadata = 0;
 static int zio_requeue_io_start_cut_in_line = 1;
 
-static int zio_dynamic_gang_headers_enable = 0;
-
 #ifdef ZFS_DEBUG
 static const int zio_buf_debug_limit = 16384;
 #else
@@ -3204,8 +3202,7 @@ zio_write_gang_block(zio_t *pio, metaslab_class_t *mc)
 	    bp, gbh_copies, txg, pio == gio ? NULL : gio->io_bp, flags,
 	    &pio->io_alloc_list, pio, pio->io_allocator);
 
-	if (spa_feature_is_enabled(spa, SPA_FEATURE_DYNAMIC_GANG_HEADER) &&
-	    zio_dynamic_gang_headers_enable) {
+	if (spa_feature_is_enabled(spa, SPA_FEATURE_DYNAMIC_GANG_HEADER)) {
 		gangblocksize = UINT64_MAX;
 		spa_config_enter(spa, SCL_VDEV, FTAG, RW_READER);
 		for (int dva = 0; dva < BP_GET_NDVAS(bp); dva++) {
@@ -5894,6 +5891,3 @@ ZFS_MODULE_PARAM(zfs_zio, zio_, dva_throttle_enabled, INT, ZMOD_RW,
 
 ZFS_MODULE_PARAM(zfs_zio, zio_, deadman_log_all, INT, ZMOD_RW,
 	"Log all slow ZIOs, not just those with vdevs");
-
-ZFS_MODULE_PARAM(zfs_zio, zio_, dynamic_gang_headers_enable, INT, ZMOD_RW,
-	"Enable dynamic gang header creation");