zarcstat: detect attached L2ARC device with no data

zarcstat and zarcsummary detected L2ARC presence using the l2_size
kstat, which is data held in L2ARC, not whether a cache device is
attached. When a cache device was attached but empty (freshly added,
or fully evicted):

  - zarcstat rejected "-f l2*" with "Incompatible field specified!"
  - zarcsummary printed "L2ARC not detected, skipping section",
    hiding cumulative I/O history and health counters

Expose the existing l2arc_ndev counter as a new kstat l2_dev_count.
It is maintained by l2arc_add_vdev() and l2arc_remove_vdev(), so it
tracks attachment in real time. Use it in both tools, falling back to
l2_size for compatibility with older kernel modules.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Signed-off-by: Ameer Hamza <ahamza@ixsystems.com>
Closes #18499

cmd/zarcstat.in

@@ -565,10 +565,10 @@ def init():
 
     update_hdr_intr()
 
-    # check if L2ARC exists
+    # check if L2ARC exists; fall back to l2_size for older kernels that
+    # do not export l2_ndev
     snap_stats()
-    l2_size = cur.get("l2_size")
-    if l2_size:
+    if cur.get("l2_ndev") or cur.get("l2_size"):
         l2exist = True
 
     if desired_cols:

cmd/zarcsummary

@@ -856,7 +856,10 @@ def section_l2arc(kstats_dict):
     # The L2ARC statistics live in the same section as the normal ARC stuff
     arc_stats = isolate_section('arcstats', kstats_dict)
 
-    if arc_stats['l2_size'] == '0':
+    # Skip the section only when no cache device is attached. Fall back to
+    # l2_size for older kernels that do not export l2_ndev.
+    if arc_stats.get('l2_ndev', '0') == '0' and \
+            arc_stats['l2_size'] == '0':
         print('L2ARC not detected, skipping section\n')
         return
 

include/sys/arc_impl.h

@@ -832,6 +832,8 @@ typedef struct arc_stats {
 	 * due to ARC_FLAG_UNCACHED being set.
 	 */
 	kstat_named_t arcstat_uncached_evictable_metadata;
+	/* Number of L2ARC devices currently attached across all pools. */
+	kstat_named_t arcstat_l2_ndev;
 	kstat_named_t arcstat_l2_hits;
 	kstat_named_t arcstat_l2_misses;
 	/*

module/zfs/arc.c

@@ -586,6 +586,7 @@ arc_stats_t arc_stats = {
 	{ "uncached_metadata",		KSTAT_DATA_UINT64 },
 	{ "uncached_evictable_data",	KSTAT_DATA_UINT64 },
 	{ "uncached_evictable_metadata", KSTAT_DATA_UINT64 },
+	{ "l2_ndev",			KSTAT_DATA_UINT64 },
 	{ "l2_hits",			KSTAT_DATA_UINT64 },
 	{ "l2_misses",			KSTAT_DATA_UINT64 },
 	{ "l2_prefetch_asize",		KSTAT_DATA_UINT64 },
@@ -7440,6 +7441,7 @@ arc_kstat_update(kstat_t *ksp, int rw)
 	    aggsum_value(&arc_sums.arcstat_dnode_size);
 	as->arcstat_bonus_size.value.ui64 =
 	    wmsum_value(&arc_sums.arcstat_bonus_size);
+	as->arcstat_l2_ndev.value.ui64 = l2arc_ndev;
 	as->arcstat_l2_hits.value.ui64 =
 	    wmsum_value(&arc_sums.arcstat_l2_hits);
 	as->arcstat_l2_misses.value.ui64 =