Enable L2 cache of all (MRU+MFU) metadata but MFU data only

man/man4/zfs.4

@@ -121,20 +121,26 @@ Controls whether buffers present on special vdevs are eligible for caching
 into L2ARC.
 If set to 1, exclude dbufs on special vdevs from being cached to L2ARC.
 .
-.It Sy l2arc_mfuonly Ns = Ns Sy 0 Ns | Ns 1 Pq  int
+.It Sy l2arc_mfuonly Ns = Ns Sy 0 Ns | Ns 1 Ns | Ns 2 Pq int
 Controls whether only MFU metadata and data are cached from ARC into L2ARC.
 This may be desired to avoid wasting space on L2ARC when reading/writing large
 amounts of data that are not expected to be accessed more than once.
 .Pp
-The default is off,
+The default is 0,
 meaning both MRU and MFU data and metadata are cached.
-When turning off this feature, some MRU buffers will still be present
-in ARC and eventually cached on L2ARC.
+When turning off this feature (setting it to 0), some MRU buffers will
+still be present in ARC and eventually cached on L2ARC.
 .No If Sy l2arc_noprefetch Ns = Ns Sy 0 ,
 some prefetched buffers will be cached to L2ARC, and those might later
 transition to MRU, in which case the
 .Sy l2arc_mru_asize No arcstat will not be Sy 0 .
 .Pp
+Setting it to 1 means to L2 cache only MFU data and metadata.
+.Pp
+Setting it to 2 means to L2 cache all metadata (MRU+MFU) but
+only MFU data (ie: MRU data are not cached). This can be the right setting
+to cache as much metadata as possible even when having high data turnover.
+.Pp
 Regardless of
 .Sy l2arc_noprefetch ,
 some MFU buffers might be evicted from ARC,

module/zfs/arc.c

@@ -9130,12 +9130,17 @@ l2arc_write_buffers(spa_t *spa, l2arc_dev_t *dev, uint64_t target_sz)
 	 */
 	for (int pass = 0; pass < L2ARC_FEED_TYPES; pass++) {
 		/*
-		 * If pass == 1 or 3, we cache MRU metadata and data
-		 * respectively.
+		 * pass == 0: MFU meta
+		 * pass == 1: MRU meta
+		 * pass == 2: MFU data
+		 * pass == 3: MRU data
 		 */
-		if (l2arc_mfuonly) {
+		if (l2arc_mfuonly == 1) {
 			if (pass == 1 || pass == 3)
 				continue;
+		} else if (l2arc_mfuonly > 1) {
+			if (pass == 3)
+				continue;
 		}
 
 		uint64_t passed_sz = 0;