Add zdb -Y for split block reconstruction

The new -Y flag allows `zdb` to try all possible combinations
when performing split block reconstruction.

Depending on the extent of the damage this may not be able to
complete in a reasonable amount of time.  However, it is primarily
intended to be used by ztest(8) which by design should never be
able to damage a pool beyond repair.  The worst case observed
has been blocks with 18 splits which can be recovered in a few
minutes.

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

cmd/zdb/zdb.c

@@ -100,6 +100,7 @@ extern int zfs_recover;
 extern uint64_t zfs_arc_max, zfs_arc_meta_limit;
 extern int zfs_vdev_async_read_max_active;
 extern boolean_t spa_load_verify_dryrun;
+extern int zfs_reconstruct_indirect_combinations_max;
 
 static const char cmdname[] = "zdb";
 uint8_t dump_opt[256];
@@ -215,6 +216,8 @@ usage(void)
 	    "dump all read blocks into specified directory\n");
 	(void) fprintf(stderr, "        -X attempt extreme rewind (does not "
 	    "work with dataset)\n");
+	(void) fprintf(stderr, "        -Y attempt all reconstruction "
+	    "combinations for split blocks\n");
 	(void) fprintf(stderr, "Specify an option more than once (e.g. -bb) "
 	    "to make only that option verbose\n");
 	(void) fprintf(stderr, "Default is to dump everything non-verbosely\n");
@@ -5871,7 +5874,7 @@ main(int argc, char **argv)
 		spa_config_path = spa_config_path_env;
 
 	while ((c = getopt(argc, argv,
-	    "AbcCdDeEFGhiI:klLmMo:Op:PqRsSt:uU:vVx:X")) != -1) {
+	    "AbcCdDeEFGhiI:klLmMo:Op:PqRsSt:uU:vVx:XY")) != -1) {
 		switch (c) {
 		case 'b':
 		case 'c':
@@ -5903,6 +5906,10 @@ main(int argc, char **argv)
 		case 'X':
 			dump_opt[c]++;
 			break;
+		case 'Y':
+			zfs_reconstruct_indirect_combinations_max = INT_MAX;
+			zfs_deadman_enabled = 0;
+			break;
 		/* NB: Sort single match options below. */
 		case 'I':
 			max_inflight = strtoull(optarg, NULL, 0);

cmd/ztest/ztest.c

@@ -6351,8 +6351,7 @@ ztest_run_zdb(char *pool)
 	ztest_get_zdb_bin(bin, len);
 
 	(void) sprintf(zdb,
-	    "%s -bcc%s%s -G -d -U %s "
-	    "-o zfs_reconstruct_indirect_combinations_max=65536 %s",
+	    "%s -bcc%s%s -G -d -Y -U %s %s",
 	    bin,
 	    ztest_opts.zo_verbose >= 3 ? "s" : "",
 	    ztest_opts.zo_verbose >= 4 ? "v" : "",

man/man8/zdb.8

@@ -23,7 +23,7 @@
 .Nd display zpool debugging and consistency information
 .Sh SYNOPSIS
 .Nm
-.Op Fl AbcdDFGhikLMPsvX
+.Op Fl AbcdDFGhikLMPsvXY
 .Op Fl e Oo Fl V Oc Op Fl p Ar path ...
 .Op Fl I Ar inflight I/Os
 .Oo Fl o Ar var Ns = Ns Ar value Oc Ns ...
@@ -50,7 +50,7 @@
 .Ar device
 .Nm
 .Fl m
-.Op Fl AFLPX
+.Op Fl AFLPXY
 .Op Fl e Oo Fl V Oc Op Fl p Ar path ...
 .Op Fl t Ar txg
 .Op Fl U Ar cache
@@ -349,6 +349,10 @@ Attempt
 transaction rewind, that is attempt the same recovery as
 .Fl F
 but read transactions otherwise deemed too old.
+.It Fl Y
+Attempt all possible combinations when reconstructing indirect split blocks.
+This flag disables the individual I/O deadman timer in order to allow as
+much time as required for the attempted reconstruction.
 .El
 .Pp
 Specifying a display option more than once enables verbosity for only that