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. Todays 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).

Signed-off-by: Gionatan Danti <[email protected]>

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