Increase L2ARC write rate and headroom

Current L2ARC write rate and headroom parameters are very conservative: l2arc_write_max=8M and l2arc_headroom=2 (ie: a full L2ARC writes at 8 MB/s, scanning 16/32 MB of ARC tail each time; a warming L2ARC runs at 2x these rates). These values were selected 15+ years ago based on then-current SSDs size, performance and endurance. Today we have multi-TB, fast and cheap SSDs which can sustain much higher read/write rates. For this reason, this patch increases l2arc_write_max to 32M and l2arc_headroom to 8 (4x increase for both). Reviewed-by: Brian Behlendorf <[email protected]> Reviewed-by: Alexander Motin <[email protected]> Signed-off-by: Gionatan Danti <[email protected]> Closes openzfs#15457
openzfsonwindows · Dec 12, 2023 · 0a7fc0b · 0a7fc0b
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diff --git a/man/man4/zfs.4 b/man/man4/zfs.4
@@ -97,7 +97,7 @@ and only applicable in related situations.
 .It Sy l2arc_feed_secs Ns = Ns Sy 1 Pq u64
 Seconds between L2ARC writing.
 .
-.It Sy l2arc_headroom Ns = Ns Sy 2 Pq u64
+.It Sy l2arc_headroom Ns = Ns Sy 8 Pq u64
 How far through the ARC lists to search for L2ARC cacheable content,
 expressed as a multiplier of
 .Sy l2arc_write_max .
@@ -200,12 +200,12 @@ to enable caching/reading prefetches to/from L2ARC.
 .It Sy l2arc_norw Ns = Ns Sy 0 Ns | Ns 1 Pq int
 No reads during writes.
 .
-.It Sy l2arc_write_boost Ns = Ns Sy 8388608 Ns B Po 8 MiB Pc Pq u64
+.It Sy l2arc_write_boost Ns = Ns Sy 33554432 Ns B Po 32 MiB Pc Pq u64
 Cold L2ARC devices will have
 .Sy l2arc_write_max
 increased by this amount while they remain cold.
 .
-.It Sy l2arc_write_max Ns = Ns Sy 8388608 Ns B Po 8 MiB Pc Pq u64
+.It Sy l2arc_write_max Ns = Ns Sy 33554432 Ns B Po 32 MiB Pc Pq u64
 Max write bytes per interval.
 .
 .It Sy l2arc_rebuild_enabled Ns = Ns Sy 1 Ns | Ns 0 Pq int

diff --git a/module/zfs/arc.c b/module/zfs/arc.c
@@ -776,8 +776,8 @@ uint64_t zfs_crc64_table[256];
  * Level 2 ARC
  */
 
-#define	L2ARC_WRITE_SIZE	(8 * 1024 * 1024)	/* initial write max */
-#define	L2ARC_HEADROOM		2			/* num of writes */
+#define	L2ARC_WRITE_SIZE	(32 * 1024 * 1024)	/* initial write max */
+#define	L2ARC_HEADROOM		8			/* num of writes */
 
 /*
  * If we discover during ARC scan any buffers to be compressed, we boost