Add tunable to allow changing micro ZAP's max size

This change turns `MZAP_MAX_BLKSZ` into a `ZFS_MODULE_PARAM()` called `zap_micro_max_size`. As a result, we can experiment with different micro ZAP sizes to improve directory size scaling. Reviewed-by: Brian Behlendorf <[email protected]> Co-authored-by: Mateusz Piotrowski <[email protected]> Co-authored-by: Toomas Soome <[email protected]> Signed-off-by: Mateusz Piotrowski <[email protected]> Sponsored-by: Wasabi Technology, Inc. Closes openzfs#14292
JKDingwall · Jan 10, 2023 · a4b21ea · a4b21ea
1 parent 1f19826
commit a4b21ea
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Showing 3 changed files with 14 additions and 3 deletions.
diff --git a/man/man4/zfs.4 b/man/man4/zfs.4
@@ -15,7 +15,7 @@
 .\" own identifying information:
 .\" Portions Copyright [yyyy] [name of copyright owner]
 .\"
-.Dd November 9, 2022
+.Dd December 21, 2022
 .Dt ZFS 4
 .Os
 .
@@ -496,6 +496,10 @@ prefetch the entire object (all leaf blocks).
 However, this is limited by
 .Sy dmu_prefetch_max .
 .
+.It Sy zap_micro_max_size Ns = Ns Sy 131072 Ns B Po 128 KiB Pc Pq int
+Maximum micro ZAP size.
+A micro ZAP is upgraded to a fat ZAP, once it grows beyond the specified size.
+.
 .It Sy zfetch_array_rd_sz Ns = Ns Sy 1048576 Ns B Po 1 MiB Pc Pq u64
 If prefetching is enabled, disable prefetching for reads larger than this size.
 .

diff --git a/module/zfs/dmu_tx.c b/module/zfs/dmu_tx.c
@@ -465,6 +465,7 @@ dmu_tx_hold_zap_impl(dmu_tx_hold_t *txh, const char *name)
 	dmu_tx_t *tx = txh->txh_tx;
 	dnode_t *dn = txh->txh_dnode;
 	int err;
+	extern int zap_micro_max_size;
 
 	ASSERT(tx->tx_txg == 0);
 
@@ -480,7 +481,7 @@ dmu_tx_hold_zap_impl(dmu_tx_hold_t *txh, const char *name)
 	 *    - 2 grown ptrtbl blocks
 	 */
 	(void) zfs_refcount_add_many(&txh->txh_space_towrite,
-	    MZAP_MAX_BLKSZ, FTAG);
+	    zap_micro_max_size, FTAG);
 
 	if (dn == NULL)
 		return;

diff --git a/module/zfs/zap_micro.c b/module/zfs/zap_micro.c
@@ -41,6 +41,8 @@
 #include <sys/sunddi.h>
 #endif
 
+int zap_micro_max_size = MZAP_MAX_BLKSZ;
+
 static int mzap_upgrade(zap_t **zapp,
     const void *tag, dmu_tx_t *tx, zap_flags_t flags);
 
@@ -568,7 +570,7 @@ zap_lockdir_impl(dmu_buf_t *db, const void *tag, dmu_tx_t *tx,
 	if (zap->zap_ismicro && tx && adding &&
 	    zap->zap_m.zap_num_entries == zap->zap_m.zap_num_chunks) {
 		uint64_t newsz = db->db_size + SPA_MINBLOCKSIZE;
-		if (newsz > MZAP_MAX_BLKSZ) {
+		if (newsz > zap_micro_max_size) {
 			dprintf("upgrading obj %llu: num_entries=%u\n",
 			    (u_longlong_t)obj, zap->zap_m.zap_num_entries);
 			*zapp = zap;
@@ -1724,4 +1726,8 @@ EXPORT_SYMBOL(zap_cursor_advance);
 EXPORT_SYMBOL(zap_cursor_serialize);
 EXPORT_SYMBOL(zap_cursor_init_serialized);
 EXPORT_SYMBOL(zap_get_stats);
+
+/* CSTYLED */
+ZFS_MODULE_PARAM(zfs, , zap_micro_max_size, INT, ZMOD_RW,
+	"Maximum micro ZAP size, before converting to a fat ZAP, in bytes");
 #endif