btrfs-progs: add safety delay before starting full balance

A short delay with a warning before starting a full balance should
improve usability. We have been getting reports from people who run full
balance after following some random advice and then get surprised by the
performance impact.

The countdown is done even when run from scripts, but as the whole
balance takes significanly more time, this shouldn't be an issue.

Signed-off-by: David Sterba <dsterba@suse.com>

Documentation/btrfs-balance.asciidoc

@@ -67,6 +67,12 @@ resume interrupted balance
 start the balance operation according to the specified filters, no filters
 will rewrite the entire filesystem. The process runs in the foreground.
 +
+NOTE: the balance command without filters will basically rewrite everything
+in the filesystem. The run time is potentially very long, depending on the
+filesystem size. To prevent starting a full balance by accident, the user is
+warned and has a few seconds to cancel the operation before it starts. The
+warning and delay can be skipped with '--full-balance' option.
++
 `Options`
 +
 -d[<filters>]::::

cmds-balance.c

@@ -417,8 +417,13 @@ static int do_balance_v1(int fd)
 	return ret;
 }
 
+enum {
+	BALANCE_START_FILTERS = 1 << 0,
+	BALANCE_START_NOWARN  = 1 << 1
+};
+
 static int do_balance(const char *path, struct btrfs_ioctl_balance_args *args,
-		      int nofilters)
+		      unsigned flags)
 {
 	int fd;
 	int ret;
@@ -429,6 +434,24 @@ static int do_balance(const char *path, struct btrfs_ioctl_balance_args *args,
 	if (fd < 0)
 		return 1;
 
+	if (!(flags & BALANCE_START_FILTERS) && !(flags & BALANCE_START_NOWARN)) {
+		int delay = 10;
+
+		printf("WARNING:\n\n");
+		printf("\tFull balance without filters requested. This operation is very\n");
+		printf("\tintense and takes potentially very long. It is recommended to\n");
+		printf("\tuse the balance filters to narrow down the balanced data.\n");
+		printf("\tUse 'btrfs balance start --full-balance' option to skip this\n");
+		printf("\twarning. The operation will start in %d seconds.\n", delay);
+		printf("\tUse Ctrl-C to stop it.\n");
+		while (delay) {
+			sleep(1);
+			printf("%2d", delay--);
+			fflush(stdout);
+		}
+		printf("\nStarting balance without any filters.\n");
+	}
+
 	ret = ioctl(fd, BTRFS_IOC_BALANCE_V2, args);
 	e = errno;
 
@@ -438,7 +461,7 @@ static int do_balance(const char *path, struct btrfs_ioctl_balance_args *args,
 		 * old one.  But, the old one doesn't know any filters, so
 		 * don't fall back if they tried to use the fancy new things
 		 */
-		if (e == ENOTTY && nofilters) {
+		if (e == ENOTTY && !(flags & BALANCE_START_FILTERS)) {
 			ret = do_balance_v1(fd);
 			if (ret == 0)
 				goto out;
@@ -477,13 +500,16 @@ static const char * const cmd_balance_start_usage[] = {
 	"passed all filters in a comma-separated list of filters for a",
 	"particular chunk type.  If filter list is not given balance all",
 	"chunks of that type.  In case none of the -d, -m or -s options is",
-	"given balance all chunks in a filesystem.",
+	"given balance all chunks in a filesystem. This is potentially",
+	"long operation and the user is warned before this start, with",
+	"a delay to stop it.",
 	"",
 	"-d[filters]    act on data chunks",
 	"-m[filters]    act on metadata chunks",
 	"-s[filters]    act on system chunks (only under -f)",
 	"-v             be verbose",
 	"-f             force reducing of metadata integrity",
+	"--full-balance do not print warning and do not delay start",
 	NULL
 };
 
@@ -494,19 +520,22 @@ static int cmd_balance_start(int argc, char **argv)
 						&args.meta, NULL };
 	int force = 0;
 	int verbose = 0;
-	int nofilters = 1;
+	unsigned start_flags = 0;
 	int i;
 
 	memset(&args, 0, sizeof(args));
 
 	optind = 1;
 	while (1) {
+		enum { GETOPT_VAL_FULL_BALANCE = 256 };
 		static const struct option longopts[] = {
 			{ "data", optional_argument, NULL, 'd'},
 			{ "metadata", optional_argument, NULL, 'm' },
 			{ "system", optional_argument, NULL, 's' },
 			{ "force", no_argument, NULL, 'f' },
 			{ "verbose", no_argument, NULL, 'v' },
+			{ "full-balance", no_argument, NULL,
+				GETOPT_VAL_FULL_BALANCE },
 			{ NULL, 0, NULL, 0 }
 		};
 
@@ -516,21 +545,21 @@ static int cmd_balance_start(int argc, char **argv)
 
 		switch (opt) {
 		case 'd':
-			nofilters = 0;
+			start_flags |= BALANCE_START_FILTERS;
 			args.flags |= BTRFS_BALANCE_DATA;
 
 			if (parse_filters(optarg, &args.data))
 				return 1;
 			break;
 		case 's':
-			nofilters = 0;
+			start_flags |= BALANCE_START_FILTERS;
 			args.flags |= BTRFS_BALANCE_SYSTEM;
 
 			if (parse_filters(optarg, &args.sys))
 				return 1;
 			break;
 		case 'm':
-			nofilters = 0;
+			start_flags |= BALANCE_START_FILTERS;
 			args.flags |= BTRFS_BALANCE_METADATA;
 
 			if (parse_filters(optarg, &args.meta))
@@ -542,6 +571,9 @@ static int cmd_balance_start(int argc, char **argv)
 		case 'v':
 			verbose = 1;
 			break;
+		case GETOPT_VAL_FULL_BALANCE:
+			start_flags |= BALANCE_START_NOWARN;
+			break;
 		default:
 			usage(cmd_balance_start_usage);
 		}
@@ -567,7 +599,7 @@ static int cmd_balance_start(int argc, char **argv)
 			sizeof(struct btrfs_balance_args));
 	}
 
-	if (nofilters) {
+	if (!(start_flags & BALANCE_START_FILTERS)) {
 		/* relocate everything - no filters */
 		args.flags |= BTRFS_BALANCE_TYPE_MASK;
 	}
@@ -595,7 +627,7 @@ static int cmd_balance_start(int argc, char **argv)
 	if (verbose)
 		dump_ioctl_balance_args(&args);
 
-	return do_balance(argv[optind], &args, nofilters);
+	return do_balance(argv[optind], &args, start_flags);
 }
 
 static const char * const cmd_balance_pause_usage[] = {
@@ -833,6 +865,16 @@ static int cmd_balance_status(int argc, char **argv)
 	return 1;
 }
 
+static int cmd_balance_full(int argc, char **argv)
+{
+	struct btrfs_ioctl_balance_args args;
+
+	memset(&args, 0, sizeof(args));
+	args.flags |= BTRFS_BALANCE_TYPE_MASK;
+
+	return do_balance(argv[1], &args, BALANCE_START_NOWARN);
+}
+
 static const char balance_cmd_group_info[] =
 "balance data accross devices, or change block groups using filters";
 
@@ -843,20 +885,21 @@ const struct cmd_group balance_cmd_group = {
 		{ "cancel", cmd_balance_cancel, cmd_balance_cancel_usage, NULL, 0 },
 		{ "resume", cmd_balance_resume, cmd_balance_resume_usage, NULL, 0 },
 		{ "status", cmd_balance_status, cmd_balance_status_usage, NULL, 0 },
+		{ "--full-balance", cmd_balance_full, NULL, NULL, 1 },
 		NULL_CMD_STRUCT
 	}
 };
 
 int cmd_balance(int argc, char **argv)
 {
-	if (argc == 2) {
+	if (argc == 2 && strcmp("start", argv[1]) != 0) {
 		/* old 'btrfs filesystem balance <path>' syntax */
 		struct btrfs_ioctl_balance_args args;
 
 		memset(&args, 0, sizeof(args));
 		args.flags |= BTRFS_BALANCE_TYPE_MASK;
 
-		return do_balance(argv[1], &args, 1);
+		return do_balance(argv[1], &args, 0);
 	}
 
 	return handle_command_group(&balance_cmd_group, argc, argv);