Set zfs_arc_min to 4MB

Decrease the mimimum ARC size from 1/32 of total system memory
(or 64MB) to a much smaller 4MB.

1) Large systems with over a 1TB of memory are being deployed
   and reserving 1/32 of this memory (32GB) as the mimimum
   requirement is overkill.

2) Tiny systems like the raspberry pi may only have 256MB of
   memory in which case 64MB is far too large.

The ARC should be reclaimable if the VFS determines it needs
the memory for some other purpose.  If you want to ensure the
ARC is never completely reclaimed due to memory pressure you
may still set a larger value with zfs_arc_min.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

module/zfs/arc.c

@@ -4025,8 +4025,8 @@ arc_init(void)
 	spl_register_shrinker(&arc_shrinker);
 #endif
 
-	/* set min cache to 1/32 of all memory, or 64MB, whichever is more */
-	arc_c_min = MAX(arc_c / 4, 64<<20);
+	/* set min cache to zero */
+	arc_c_min = 4<<20;
 	/* set max to 1/2 of all memory */
 	arc_c_max = arc_c * 4;
 
@@ -4036,7 +4036,7 @@ arc_init(void)
 	 */
 	if (zfs_arc_max > 64<<20 && zfs_arc_max < physmem * PAGESIZE)
 		arc_c_max = zfs_arc_max;
-	if (zfs_arc_min > 64<<20 && zfs_arc_min <= arc_c_max)
+	if (zfs_arc_min > 0 && zfs_arc_min <= arc_c_max)
 		arc_c_min = zfs_arc_min;
 
 	arc_c = arc_c_max;
@@ -4050,9 +4050,6 @@ arc_init(void)
 	if (zfs_arc_meta_limit > 0 && zfs_arc_meta_limit <= arc_c_max)
 		arc_meta_limit = zfs_arc_meta_limit;
 
-	if (arc_c_min < arc_meta_limit / 2 && zfs_arc_min == 0)
-		arc_c_min = arc_meta_limit / 2;
-
 	/* if kmem_flags are set, lets try to use less memory */
 	if (kmem_debugging())
 		arc_c = arc_c / 2;