L2ARC: Implement DWPD-based rate limiting with adaptive feed intervals

Add DWPD (Drive Writes Per Day) rate limiting to control L2ARC write
speeds and protect SSD endurance. Write rate is constrained by the
minimum of l2arc_write_max and DWPD-calculated budget. Devices
accumulate unused write budget over 24-hour periods with automatic reset
and carry-over. Writes occur in controlled bursts (max 50MB) with
adaptive intervals to achieve target rates. Applies after initial device
fill.

Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ameer Hamza <ahamza@ixsystems.com>
Closes #18093

module/os/freebsd/zfs/sysctl_os.c

@@ -163,6 +163,22 @@ param_set_arc_int(SYSCTL_HANDLER_ARGS)
 	return (0);
 }
 
+int
+param_set_l2arc_dwpd_limit(SYSCTL_HANDLER_ARGS)
+{
+	uint64_t old_val = l2arc_dwpd_limit;
+	int err;
+
+	err = sysctl_handle_64(oidp, arg1, 0, req);
+	if (err != 0 || req->newptr == NULL)
+		return (err);
+
+	if (l2arc_dwpd_limit != old_val)
+		l2arc_dwpd_bump_reset();
+
+	return (0);
+}
+
 int
 param_set_arc_max(SYSCTL_HANDLER_ARGS)
 {

module/os/linux/zfs/arc_os.c

@@ -410,6 +410,22 @@ param_set_arc_int(const char *buf, zfs_kernel_param_t *kp)
 	return (0);
 }
 
+int
+param_set_l2arc_dwpd_limit(const char *buf, zfs_kernel_param_t *kp)
+{
+	uint64_t old_val = l2arc_dwpd_limit;
+	int error;
+
+	error = spl_param_set_u64(buf, kp);
+	if (error < 0)
+		return (SET_ERROR(error));
+
+	if (l2arc_dwpd_limit != old_val)
+		l2arc_dwpd_bump_reset();
+
+	return (0);
+}
+
 #ifdef CONFIG_MEMORY_HOTPLUG
 static int
 arc_hotplug_callback(struct notifier_block *self, unsigned long action,