diff --git a/bcachefs.c b/bcachefs.c
index 79533411..8408c297 100644
--- a/bcachefs.c
+++ b/bcachefs.c
@@ -51,6 +51,7 @@ static void usage(void)
 	     "  device offline       Take a device offline, without removing it\n"
 	     "  device evacuate      Migrate data off of a specific device\n"
 	     "  device set-state     Mark a device as failed\n"
+	     "  device resize        Resize filesystem on a device\n"
 	     "\n"
 	     "Encryption:\n"
 	     "  unlock               Unlock an encrypted filesystem prior to running/mounting\n"
@@ -112,6 +113,8 @@ static int device_cmds(int argc, char *argv[])
 		return cmd_device_offline(argc, argv);
 	if (!strcmp(cmd, "set-state"))
 		return cmd_device_set_state(argc, argv);
+	if (!strcmp(cmd, "resize"))
+		return cmd_device_resize(argc, argv);
 
 	usage();
 	return 0;
diff --git a/cmd_assemble.c b/cmd_assemble.c
index 5fbabdd9..b2a832e6 100644
--- a/cmd_assemble.c
+++ b/cmd_assemble.c
@@ -9,6 +9,7 @@
 
 #include "libbcachefs/bcachefs_ioctl.h"
 #include "cmds.h"
+#include "libbcachefs.h"
 
 int cmd_assemble(int argc, char *argv[])
 {
diff --git a/cmd_device.c b/cmd_device.c
index f2d751c5..f8e0cfbf 100644
--- a/cmd_device.c
+++ b/cmd_device.c
@@ -13,6 +13,7 @@
 #include <unistd.h>
 
 #include "libbcachefs/bcachefs_ioctl.h"
+#include "libbcachefs/super-io.h"
 #include "cmds.h"
 #include "libbcachefs.h"
 #include "libbcachefs/opts.h"
@@ -99,7 +100,7 @@ int cmd_device_show(int argc, char *argv[])
 	if (argc != 2)
 		die("Please supply a single device");
 
-	struct bcache_handle fs = bcache_fs_open(argv[1]);
+	struct bchfs_handle fs = bcache_fs_open(argv[1]);
 	struct dirent *entry;
 
 	struct bcache_dev devices[256];
@@ -240,7 +241,7 @@ int cmd_device_add(int argc, char *argv[])
 	if (argc - optind != 2)
 		die("Please supply a filesystem and a device to add");
 
-	struct bcache_handle fs = bcache_fs_open(argv[optind]);
+	struct bchfs_handle fs = bcache_fs_open(argv[optind]);
 
 	dev_opts.path = argv[optind + 1];
 	dev_opts.fd = open_for_format(dev_opts.path, force);
@@ -255,9 +256,7 @@ int cmd_device_add(int argc, char *argv[])
 	fsync(dev_opts.fd);
 	close(dev_opts.fd);
 
-	struct bch_ioctl_disk i = { .dev = (__u64) dev_opts.path, };
-
-	xioctl(fs.ioctl_fd, BCH_IOCTL_DISK_ADD, &i);
+	bchu_disk_add(fs, dev_opts.path);
 	return 0;
 }
 
@@ -436,20 +435,113 @@ int cmd_device_set_state(int argc, char *argv[])
 	if (argc - optind != 3)
 		die("Please supply a filesystem, device and state");
 
-	struct bcache_handle fs = bcache_fs_open(argv[optind]);
+	struct bchfs_handle fs = bcache_fs_open(argv[optind]);
 
-	struct bch_ioctl_disk_set_state i = {
-		.flags		= flags,
-		.new_state	= read_string_list_or_die(argv[optind + 2],
-						bch2_dev_state, "device state"),
-	};
-
-	const char *dev = argv[optind + 1];
-	if (!kstrtoull(dev, 10, &i.dev))
-		i.flags |= BCH_BY_INDEX;
-	else
-		i.dev = (u64) dev;
-
-	xioctl(fs.ioctl_fd, BCH_IOCTL_DISK_SET_STATE, &i);
+	bchu_disk_set_state(fs, argv[optind + 1],
+			    read_string_list_or_die(argv[optind + 2],
+					bch2_dev_state, "device state"),
+			    flags);
+	return 0;
+}
+
+static void device_resize_usage(void)
+{
+	puts("bcachefs device resize \n"
+	     "Usage: bcachefs device resize device [ size ]\n"
+	     "\n"
+	     "Options:\n"
+	     "  -h, --help                  display this help and exit\n"
+	     "Report bugs to <linux-bcache@vger.kernel.org>");
+	exit(EXIT_SUCCESS);
+}
+
+int cmd_device_resize(int argc, char *argv[])
+{
+	static const struct option longopts[] = {
+		{ "help",			0, NULL, 'h' },
+		{ NULL }
+	};
+	u64 size;
+	int opt;
+
+	while ((opt = getopt_long(argc, argv, "h", longopts, NULL)) != -1)
+		switch (opt) {
+		case 'h':
+			device_resize_usage();
+		}
+
+	if (argc < optind + 1)
+		die("Please supply a device to resize");
+
+	char *dev = argv[optind];
+	int dev_fd = xopen(dev, O_RDONLY);
+
+	if (argc == optind + 1)
+		size = get_size(dev, dev_fd);
+	else if (bch2_strtoull_h(argv[optind + 1], &size))
+		die("invalid size");
+
+	size >>= 9;
+
+	if (argc > optind + 2)
+		die("Too many arguments");
+
+	struct stat dev_stat = xfstat(dev_fd);
+
+	char *mount = dev_to_mount(dev);
+	if (mount) {
+		if (!S_ISBLK(dev_stat.st_mode))
+			die("%s is mounted but isn't a block device?!", dev);
+
+		printf("Doing online resize of %s\n", dev);
+
+		struct bchfs_handle fs = bcache_fs_open(mount);
+
+		unsigned idx = bchu_disk_get_idx(fs, dev_stat.st_rdev);
+
+		struct bch_sb *sb = bchu_read_super(fs, -1);
+		if (idx >= sb->nr_devices)
+			die("error reading superblock: dev idx >= sb->nr_devices");
+
+		struct bch_sb_field_members *mi = bch2_sb_get_members(sb);
+		if (!mi)
+			die("error reading superblock: no member info");
+
+		/* could also just read this out of sysfs... meh */
+		struct bch_member *m = mi->members + idx;
+
+		u64 nbuckets = size / le16_to_cpu(m->bucket_size);
+
+		printf("resizing %s to %llu buckets\n", dev, nbuckets);
+		bchu_disk_resize(fs, idx, nbuckets);
+	} else {
+		printf("Doing offline resize of %s\n", dev);
+
+		struct bch_fs *c = NULL;
+		struct bch_opts opts = bch2_opts_empty();
+		const char *err = bch2_fs_open(&dev, 1, opts, &c);
+		if (err)
+			die("error opening %s: %s", argv[optind], err);
+
+		struct bch_dev *ca, *resize = NULL;
+		unsigned i;
+
+		for_each_online_member(ca, c, i) {
+			if (resize)
+				die("confused: more than one online device?");
+			resize = ca;
+			percpu_ref_get(&resize->io_ref);
+		}
+
+		u64 nbuckets = size / le16_to_cpu(resize->mi.bucket_size);
+
+		printf("resizing %s to %llu buckets\n", dev, nbuckets);
+		int ret = bch2_dev_resize(c, resize, nbuckets);
+		if (ret)
+			fprintf(stderr, "resize error: %s\n", strerror(-ret));
+
+		percpu_ref_put(&resize->io_ref);
+		bch2_fs_stop(c);
+	}
 	return 0;
 }
diff --git a/cmd_fs.c b/cmd_fs.c
index 65ee539e..3f64161f 100644
--- a/cmd_fs.c
+++ b/cmd_fs.c
@@ -8,6 +8,7 @@
 #include "libbcachefs/opts.h"
 
 #include "cmds.h"
+#include "libbcachefs.h"
 
 static inline int printf_pad(unsigned pad, const char * fmt, ...)
 {
@@ -26,21 +27,11 @@ static inline int printf_pad(unsigned pad, const char * fmt, ...)
 
 static void print_fs_usage(const char *path, enum units units)
 {
-	unsigned i, j, nr_devices = 4;
-	struct bcache_handle fs = bcache_fs_open(path);
-	struct bch_ioctl_usage *u = NULL;
+	unsigned i, j;
 	char uuid[40];
 
-	while (1) {
-		u = xrealloc(u, sizeof(*u) + sizeof(u->devs[0]) * nr_devices);
-		u->nr_devices = nr_devices;
-
-		if (!ioctl(fs.ioctl_fd, BCH_IOCTL_USAGE, u))
-			break;
-		if (errno != ENOSPC)
-			die("BCH_IOCTL_USAGE error: %m");
-		nr_devices *= 2;
-	}
+	struct bchfs_handle fs = bcache_fs_open(path);
+	struct bch_ioctl_usage *u = bchu_usage(fs);
 
 	uuid_unparse(fs.uuid.b, uuid);
 	printf("Filesystem %s:\n", uuid);
diff --git a/cmd_run.c b/cmd_run.c
index bbb37e5d..673d519a 100644
--- a/cmd_run.c
+++ b/cmd_run.c
@@ -13,6 +13,7 @@
 
 #include "libbcachefs/bcachefs_ioctl.h"
 #include "cmds.h"
+#include "libbcachefs.h"
 
 int cmd_run(int argc, char *argv[])
 {
@@ -24,7 +25,7 @@ int cmd_stop(int argc, char *argv[])
 	if (argc != 2)
 		die("Please supply a filesystem");
 
-	struct bcache_handle fs = bcache_fs_open(argv[1]);
+	struct bchfs_handle fs = bcache_fs_open(argv[1]);
 	xioctl(fs.ioctl_fd, BCH_IOCTL_STOP);
 	return 0;
 }
diff --git a/cmds.h b/cmds.h
index 1c07e072..e28a5045 100644
--- a/cmds.h
+++ b/cmds.h
@@ -25,6 +25,7 @@ int cmd_device_online(int argc, char *argv[]);
 int cmd_device_offline(int argc, char *argv[]);
 int cmd_device_evacuate(int argc, char *argv[]);
 int cmd_device_set_state(int argc, char *argv[]);
+int cmd_device_resize(int argc, char *argv[]);
 
 int cmd_unlock(int argc, char *argv[]);
 int cmd_set_passphrase(int argc, char *argv[]);
diff --git a/libbcachefs.c b/libbcachefs.c
index 351eb31e..afbc8d7a 100644
--- a/libbcachefs.c
+++ b/libbcachefs.c
@@ -455,3 +455,57 @@ void bch2_super_print(struct bch_sb *sb, int units)
 		       BCH_MEMBER_DISCARD(m));
 	}
 }
+
+/* ioctl interface: */
+
+/* Global control device: */
+int bcachectl_open(void)
+{
+	return xopen("/dev/bcachefs-ctl", O_RDWR);
+}
+
+/* Filesystem handles (ioctl, sysfs dir): */
+
+#define SYSFS_BASE "/sys/fs/bcachefs/"
+
+void bcache_fs_close(struct bchfs_handle fs)
+{
+	close(fs.ioctl_fd);
+	close(fs.sysfs_fd);
+}
+
+struct bchfs_handle bcache_fs_open(const char *path)
+{
+	struct bchfs_handle ret;
+
+	if (!uuid_parse(path, ret.uuid.b)) {
+		/* It's a UUID, look it up in sysfs: */
+		char *sysfs = mprintf("%s%s", SYSFS_BASE, path);
+		ret.sysfs_fd = xopen(sysfs, O_RDONLY);
+
+		char *minor = read_file_str(ret.sysfs_fd, "minor");
+		char *ctl = mprintf("/dev/bcachefs%s-ctl", minor);
+		ret.ioctl_fd = xopen(ctl, O_RDWR);
+
+		free(sysfs);
+		free(minor);
+		free(ctl);
+	} else {
+		/* It's a path: */
+		ret.ioctl_fd = xopen(path, O_RDONLY);
+
+		struct bch_ioctl_query_uuid uuid;
+		xioctl(ret.ioctl_fd, BCH_IOCTL_QUERY_UUID, &uuid);
+
+		ret.uuid = uuid.uuid;
+
+		char uuid_str[40];
+		uuid_unparse(uuid.uuid.b, uuid_str);
+
+		char *sysfs = mprintf("%s%s", SYSFS_BASE, uuid_str);
+		ret.sysfs_fd = xopen(sysfs, O_RDONLY);
+		free(sysfs);
+	}
+
+	return ret;
+}
diff --git a/libbcachefs.h b/libbcachefs.h
index 836cb7a9..97f4b2d8 100644
--- a/libbcachefs.h
+++ b/libbcachefs.h
@@ -5,6 +5,7 @@
 #include <stdbool.h>
 
 #include "libbcachefs/bcachefs_format.h"
+#include "libbcachefs/bcachefs_ioctl.h"
 #include "tools-util.h"
 #include "libbcachefs/vstructs.h"
 
@@ -76,4 +77,103 @@ struct bch_sb *__bch2_super_read(int, u64);
 
 void bch2_super_print(struct bch_sb *, int);
 
+/* ioctl interface: */
+
+int bcachectl_open(void);
+
+struct bchfs_handle {
+	uuid_le	uuid;
+	int	ioctl_fd;
+	int	sysfs_fd;
+};
+
+void bcache_fs_close(struct bchfs_handle);
+struct bchfs_handle bcache_fs_open(const char *);
+
+static inline void bchu_disk_add(struct bchfs_handle fs, char *dev)
+{
+	struct bch_ioctl_disk i = { .dev = (__u64) dev, };
+
+	xioctl(fs.ioctl_fd, BCH_IOCTL_DISK_ADD, &i);
+}
+
+static inline void bchu_disk_set_state(struct bchfs_handle fs, const char *dev,
+				       unsigned new_state, unsigned flags)
+{
+	struct bch_ioctl_disk_set_state i = {
+		.flags		= flags,
+		.new_state	= new_state,
+	};
+
+	if (!kstrtoull(dev, 10, &i.dev))
+		i.flags |= BCH_BY_INDEX;
+	else
+		i.dev = (u64) dev;
+
+	xioctl(fs.ioctl_fd, BCH_IOCTL_DISK_SET_STATE, &i);
+}
+
+static inline struct bch_ioctl_usage *bchu_usage(struct bchfs_handle fs)
+{
+	struct bch_ioctl_usage *u = NULL;
+	unsigned nr_devices = 4;
+
+	while (1) {
+		u = xrealloc(u, sizeof(*u) + sizeof(u->devs[0]) * nr_devices);
+		u->nr_devices = nr_devices;
+
+		if (!ioctl(fs.ioctl_fd, BCH_IOCTL_USAGE, u))
+			return u;
+
+		if (errno != ENOSPC)
+			die("BCH_IOCTL_USAGE error: %m");
+		nr_devices *= 2;
+	}
+}
+
+static inline struct bch_sb *bchu_read_super(struct bchfs_handle fs, unsigned idx)
+{
+	size_t size = 4096;
+	struct bch_sb *sb = NULL;
+
+	while (1) {
+		sb = xrealloc(sb, size);
+		struct bch_ioctl_read_super i = {
+			.size	= size,
+			.sb	= (u64) sb,
+		};
+
+		if (idx != -1) {
+			i.flags |= BCH_READ_DEV|BCH_BY_INDEX;
+			i.dev = idx;
+		}
+
+		if (!ioctl(fs.ioctl_fd, BCH_IOCTL_READ_SUPER, &i))
+			return sb;
+		if (errno != ERANGE)
+			die("BCH_IOCTL_READ_SUPER error: %m");
+		size *= 2;
+	}
+}
+
+static inline unsigned bchu_disk_get_idx(struct bchfs_handle fs, dev_t dev)
+{
+	struct bch_ioctl_disk_get_idx i = { .dev = dev };
+
+	return xioctl(fs.ioctl_fd, BCH_IOCTL_DISK_GET_IDX, &i);
+}
+
+static inline void bchu_disk_resize(struct bchfs_handle fs,
+				    unsigned idx,
+				    u64 nbuckets)
+{
+	struct bch_ioctl_disk_resize i = {
+		.flags	= BCH_BY_INDEX,
+		.dev	= idx,
+		.nbuckets = nbuckets,
+	};
+
+	xioctl(fs.ioctl_fd, BCH_IOCTL_DISK_RESIZE, &i);
+}
+
 #endif /* _LIBBCACHE_H */
diff --git a/tools-util.c b/tools-util.c
index 70f5558b..d5450495 100644
--- a/tools-util.c
+++ b/tools-util.c
@@ -262,58 +262,6 @@ int open_for_format(const char *dev, bool force)
 	return fd;
 }
 
-/* Global control device: */
-int bcachectl_open(void)
-{
-	return xopen("/dev/bcachefs-ctl", O_RDWR);
-}
-
-/* Filesystem handles (ioctl, sysfs dir): */
-
-#define SYSFS_BASE "/sys/fs/bcachefs/"
-
-void bcache_fs_close(struct bcache_handle fs)
-{
-	close(fs.ioctl_fd);
-	close(fs.sysfs_fd);
-}
-
-struct bcache_handle bcache_fs_open(const char *path)
-{
-	struct bcache_handle ret;
-
-	if (!uuid_parse(path, ret.uuid.b)) {
-		/* It's a UUID, look it up in sysfs: */
-		char *sysfs = mprintf("%s%s", SYSFS_BASE, path);
-		ret.sysfs_fd = xopen(sysfs, O_RDONLY);
-
-		char *minor = read_file_str(ret.sysfs_fd, "minor");
-		char *ctl = mprintf("/dev/bcachefs%s-ctl", minor);
-		ret.ioctl_fd = xopen(ctl, O_RDWR);
-
-		free(sysfs);
-		free(minor);
-		free(ctl);
-	} else {
-		/* It's a path: */
-		ret.ioctl_fd = xopen(path, O_RDONLY);
-
-		struct bch_ioctl_query_uuid uuid;
-		xioctl(ret.ioctl_fd, BCH_IOCTL_QUERY_UUID, &uuid);
-
-		ret.uuid = uuid.uuid;
-
-		char uuid_str[40];
-		uuid_unparse(uuid.uuid.b, uuid_str);
-
-		char *sysfs = mprintf("%s%s", SYSFS_BASE, uuid_str);
-		ret.sysfs_fd = xopen(sysfs, O_RDONLY);
-		free(sysfs);
-	}
-
-	return ret;
-}
-
 bool ask_yn(void)
 {
 	const char *short_yes = "yY";
@@ -586,9 +534,9 @@ u32 crc32c(u32 crc, const void *buf, size_t size)
 
 #endif /* HAVE_WORKING_IFUNC */
 
-char *dev_to_path(dev_t dev)
+char *dev_to_name(dev_t dev)
 {
-	char *line = NULL, *name = NULL, *path = NULL;
+	char *line = NULL, *name = NULL;
 	size_t n = 0;
 
 	FILE *f = fopen("/proc/partitions", "r");
@@ -602,17 +550,58 @@ char *dev_to_path(dev_t dev)
 		name = realloc(name, n + 1);
 
 		if (sscanf(line, " %u %u %llu %s", &ma, &mi, &sectors, name) == 4 &&
-		    ma == major(dev) && mi == minor(dev)) {
-			path = mprintf("/dev/%s", name);
-			break;
-		}
-
+		    ma == major(dev) && mi == minor(dev))
+			goto found;
 	}
 
+	free(name);
+	name = NULL;
+found:
 	fclose(f);
 	free(line);
+	return name;
+}
+
+char *dev_to_path(dev_t dev)
+{
+	char *name = dev_to_name(dev);
+	if (!name)
+		return NULL;
+
+	char *path = mprintf("/dev/%s", name);
+
 	free(name);
 	return path;
 }
 
+char *dev_to_mount(char *dev)
+{
+	char *line = NULL, *ret = NULL;
+	size_t n = 0;
+
+	FILE *f = fopen("/proc/mounts", "r");
+	if (!f)
+		die("error opening /proc/mounts: %m");
+
+	while (getline(&line, &n, f) != -1) {
+		char *d, *p = line;
+		char *devs = strsep(&p, " ");
+		char *mount = strsep(&p, " ");
+
+		if (!devs || !mount)
+			continue;
+
+		p = devs;
+		while ((d = strsep(&p, ":")))
+			if (!strcmp(d, dev)) {
+				ret = strdup(mount);
+				goto found;
+			}
+	}
+found:
+	fclose(f);
+	free(line);
+	return ret;
+}
+
 #endif
diff --git a/tools-util.h b/tools-util.h
index 649ea9bb..dcca376b 100644
--- a/tools-util.h
+++ b/tools-util.h
@@ -5,6 +5,7 @@
 #include <stdbool.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <sys/ioctl.h>
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <unistd.h>
@@ -39,10 +40,12 @@ struct stat xfstat(int);
 #define xopen(...)	xopenat(AT_FDCWD, __VA_ARGS__)
 
 #define xioctl(_fd, _nr, ...)						\
-do {									\
-	if (ioctl((_fd), (_nr), ##__VA_ARGS__))				\
+({									\
+	int _ret = ioctl((_fd), (_nr), ##__VA_ARGS__);			\
+	if (_ret < 0)							\
 		die(#_nr " ioctl error: %m");				\
-} while (0)
+	_ret;								\
+})
 
 enum units {
 	BYTES,
@@ -68,17 +71,6 @@ u64 get_size(const char *, int);
 unsigned get_blocksize(const char *, int);
 int open_for_format(const char *, bool);
 
-int bcachectl_open(void);
-
-struct bcache_handle {
-	uuid_le	uuid;
-	int	ioctl_fd;
-	int	sysfs_fd;
-};
-
-void bcache_fs_close(struct bcache_handle);
-struct bcache_handle bcache_fs_open(const char *);
-
 bool ask_yn(void);
 
 struct range {
@@ -155,6 +147,8 @@ unsigned hatoi_validate(const char *, const char *);
 
 u32 crc32c(u32, const void *, size_t);
 
+char *dev_to_name(dev_t);
 char *dev_to_path(dev_t);
+char *dev_to_mount(char *);
 
 #endif /* _TOOLS_UTIL_H */