Turn off log block writing when L2ARC size < 1GB

Signed-off-by: George Amanakis <gamanakis@gmail.com>

man/man5/zfs-module-parameters.5

@@ -256,6 +256,20 @@ fragmented/unusable).
 Use \fB1\fR for yes (default) and \fB0\fR for no.
 .RE
 
+.sp
+.ne 2
+.na
+\fBl2arc_rebuild_blocks_min_size\fR (int)
+.ad
+.RS 12n
+Min size in bytes of an L2ARC device to write rebuild log blocks in it.
+Rationale: for L2ARC devices less than 1GB, the amount of data l2arc_evict()
+evicts is significant compared to the amount of restored L2ARC data. In this
+case do not write or restore log blocks in L2ARC in order not to waste space.
+.sp
+Default value: \fB1,073,741,824\fR (1GB).
+.RE
+
 .sp
 .ne 2
 .na

module/zfs/arc.c

@@ -864,11 +864,18 @@ static void l2arc_read_done(zio_t *);
 /*
  * Performance tuning of L2ARC persistence:
  *
- * l2arc_rebuild_enabled : A ZFS module that controls whether adding an L2ARC
- * 		device (either at pool import or when adding one manually later)
- * 		will attempt to rebuild L2ARC buffer contents.
+ * l2arc_rebuild_enabled : A ZFS module parameter that controls whether adding
+ * 		an L2ARC device (either at pool import or later) will attempt
+ * 		to rebuild L2ARC buffer contents.
+ * l2arc_rebuild_blocks_min_size : A ZFS module parameter that controls whether
+ * 		log blocks are written to the L2ARC device. If the L2ARC device
+ * 		is less than 1GB, the amount of data l2arc_evict() evicts is
+ * 		significant compared to the amount of restored L2ARC data. In
+ * 		this case do not write or restore log blocks in L2ARC so as not
+ * 		to waste space.
  */
 int l2arc_rebuild_enabled = B_TRUE;
+int l2arc_rebuild_blocks_min_size = 1024 * 1024 * 1024;
 
 /* L2ARC persistence rebuild control routines. */
 static void l2arc_dev_rebuild_start(l2arc_dev_t *dev);
@@ -8690,9 +8697,10 @@ l2arc_write_buffers(spa_t *spa, l2arc_dev_t *dev, uint64_t target_sz)
 
 	/*
 	 * If we wrote any logs as part of this write, update dev hdr
-	 * to point to it.
+	 * to point to it. If log_entries = 0 also update the header here,
+	 * otherwise it will not be updated, leading to dh_errors.
 	 */
-	if (dev_hdr_update)
+	if (dev_hdr_update || dev->l2ad_dev_hdr->dh_log_blk_ent == 0)
 		l2arc_dev_hdr_update(dev, pio, cb);
 
 	/*
@@ -8850,8 +8858,9 @@ l2arc_vdev_get(vdev_t *vd)
 void
 l2arc_add_vdev(spa_t *spa, vdev_t *vd, boolean_t rebuild)
 {
-	l2arc_dev_t *adddev;
-	l2arc_dev_hdr_phys_t *hdr;
+	l2arc_dev_t		*adddev;
+	l2arc_dev_hdr_phys_t	*hdr;
+	uint64_t		log_entries;
 
 	ASSERT(!l2arc_vdev_present(vd));
 
@@ -8895,8 +8904,13 @@ l2arc_add_vdev(spa_t *spa, vdev_t *vd, boolean_t rebuild)
 	 * is less than that, we reduce the amount of committed and restored
 	 * log entries per block so as to enable persistence.
 	 */
-	uint64_t log_entries = MIN((adddev->l2ad_end - adddev->l2ad_start) >>
-	    SPA_MAXBLOCKSHIFT, L2ARC_LOG_BLK_MAX_ENTRIES);
+	if (adddev->l2ad_end < l2arc_rebuild_blocks_min_size) {
+		log_entries = 0;
+	} else {
+		log_entries = MIN((adddev->l2ad_end -
+		    adddev->l2ad_start) >> SPA_MAXBLOCKSHIFT,
+		    L2ARC_LOG_BLK_MAX_ENTRIES);
+	}
 
 	/*
 	 * Read the device header, and if hdr->dh_log_blk_ent is not equal to
@@ -9893,6 +9907,9 @@ ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, norw, INT, ZMOD_RW,
 ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, rebuild_enabled, INT, ZMOD_RW,
 	"Rebuild the L2ARC when importing a pool");
 
+ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, rebuild_blocks_min_size, INT, ZMOD_RW,
+	"Min size in bytes to write rebuild log blocks in L2ARC");
+
 ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, lotsfree_percent, param_set_arc_int,
 	param_get_int, ZMOD_RW, "System free memory I/O throttle in bytes");
 

tests/zfs-tests/tests/functional/persist_l2arc/persist_l2arc_001_pos.ksh

@@ -59,12 +59,16 @@ function cleanup
 	fi
 
 	log_must set_tunable32 l2arc_noprefetch $noprefetch
+	log_must set_tunable32 l2arc_rebuild_blocks_min_size \
+		$rebuild_blocks_min_size
 }
 log_onexit cleanup
 
 # l2arc_noprefetch is set to 0 to let L2ARC handle prefetches
 typeset noprefetch=$(get_tunable l2arc_noprefetch)
+typeset rebuild_blocks_min_size=$(get_tunable l2arc_rebuild_blocks_min_size)
 log_must set_tunable32 l2arc_noprefetch 0
+log_must set_tunable32 l2arc_rebuild_blocks_min_size 0
 
 typeset fill_mb=800
 typeset cache_sz=$(( floor($fill_mb / 2 ) ))
@@ -90,7 +94,8 @@ log_must fio $FIO_SCRIPTS/random_reads.fio
 log_must zpool export $TESTPOOL
 log_must zpool import -d $VDIR $TESTPOOL
 
-log_must test "$(zpool iostat -Hpv $TESTPOOL $VDEV_CACHE | awk '{print $2}')" -gt 23702188
+log_must test "$(zpool iostat -Hpv $TESTPOOL $VDEV_CACHE | awk '{print $2}')" \
+	-gt 23702188
 
 typeset l2_hits_start=$(grep l2_hits /proc/spl/kstat/zfs/arcstats | \
 	awk '{print $3}')

tests/zfs-tests/tests/functional/persist_l2arc/persist_l2arc_002_pos.ksh

@@ -63,12 +63,16 @@ function cleanup
 	fi
 
 	log_must set_tunable32 l2arc_noprefetch $noprefetch
+	log_must set_tunable32 l2arc_rebuild_blocks_min_size \
+		$rebuild_blocks_min_size
 }
 log_onexit cleanup
 
 # l2arc_noprefetch is set to 0 to let L2ARC handle prefetches
 typeset noprefetch=$(get_tunable l2arc_noprefetch)
+typeset rebuild_blocks_min_size=$(get_tunable l2arc_rebuild_blocks_min_size)
 log_must set_tunable32 l2arc_noprefetch 0
+log_must set_tunable32 l2arc_rebuild_blocks_min_size 0
 
 typeset fill_mb=800
 typeset cache_sz=$(( floor($fill_mb / 2) ))
@@ -97,7 +101,8 @@ log_must fio $FIO_SCRIPTS/random_reads.fio
 log_must zpool export $TESTPOOL
 log_must zpool import -d $VDIR $TESTPOOL
 log_must eval "echo $PASSPHRASE | zfs mount -l $TESTPOOL/$TESTFS1"
-log_must test "$(zpool iostat -Hpv $TESTPOOL $VDEV_CACHE | awk '{print $2}')" -gt 23702188
+log_must test "$(zpool iostat -Hpv $TESTPOOL $VDEV_CACHE | awk '{print $2}')" \
+	-gt 23702188
 
 typeset l2_hits_start=$(grep l2_hits /proc/spl/kstat/zfs/arcstats | \
 	awk '{print $3}')