zdb: add extra -T flag to show histograms of BRT refcounts

Sponsored-by: https://despairlabs.com/sponsor/
Signed-off-by: Rob Norris <[email protected]>

Author: robn

cmd/zdb/zdb.c

@@ -2152,14 +2152,21 @@ dump_brt(spa_t *spa)
 	if (dump_opt['T'] < 3)
 		return;
 
+	/* -TTT shows a per-vdev histograms; -TTTT shows all entries */
+	boolean_t do_histo = dump_opt['T'] == 3;
+
 	char dva[64];
-	printf("\n%-16s %-10s\n", "DVA", "REFCNT");
+
+	if (!do_histo)
+		printf("\n%-16s %-10s\n", "DVA", "REFCNT");
 
 	for (uint64_t vdevid = 0; vdevid < brt->brt_nvdevs; vdevid++) {
 		brt_vdev_t *brtvd = &brt->brt_vdevs[vdevid];
 		if (brtvd == NULL || !brtvd->bv_initiated)
 			continue;
 
+		uint64_t counts[64] = {};
+
 		zap_cursor_t zc;
 		zap_attribute_t *za = zap_attribute_alloc();
 		for (zap_cursor_init(&zc, brt->brt_mos, brtvd->bv_mos_entries);
@@ -2172,14 +2179,26 @@ dump_brt(spa_t *spa)
 			    za->za_integer_length, za->za_num_integers,
 			    &refcnt));
 
-			uint64_t offset = *(const uint64_t *)za->za_name;
+			if (do_histo)
+				counts[highbit64(refcnt)]++;
+			else {
+				uint64_t offset =
+				    *(const uint64_t *)za->za_name;
 
-			snprintf(dva, sizeof (dva), "%" PRIu64 ":%llx", vdevid,
-			    (u_longlong_t)offset);
-			printf("%-16s %-10llu\n", dva, (u_longlong_t)refcnt);
+				snprintf(dva, sizeof (dva), "%" PRIu64 ":%llx",
+				    vdevid, (u_longlong_t)offset);
+				printf("%-16s %-10llu\n", dva,
+				    (u_longlong_t)refcnt);
+			}
 		}
 		zap_cursor_fini(&zc);
 		zap_attribute_free(za);
+
+		if (do_histo) {
+			printf("\nBRT: vdev %" PRIu64
+			    ": DVAs with 2^n refcnts:\n", vdevid);
+			dump_histogram(counts, 64, 0);
+		}
 	}
 }
 

man/man8/zdb.8

@@ -14,7 +14,7 @@
 .\" Copyright (c) 2017 Lawrence Livermore National Security, LLC.
 .\" Copyright (c) 2017 Intel Corporation.
 .\"
-.Dd November 18, 2023
+.Dd October 27, 2024
 .Dt ZDB 8
 .Os
 .
@@ -408,6 +408,8 @@ blocks cloned, the space saving as a result of cloning, and the saving ratio.
 .It Fl TT
 Display the per-vdev BRT statistics, including total references.
 .It Fl TTT
+Display histograms of per-vdev BRT refcounts.
+.It Fl TTTT
 Dump the contents of the block reference tables.
 .It Fl u , -uberblock
 Display the current uberblock.