diff --git a/f3probe.c b/f3probe.c
index d6204ce..8749170 100644
--- a/f3probe.c
+++ b/f3probe.c
@@ -43,6 +43,8 @@ static struct argp_option options[] = {
 		"Run a unit test; it ignores all other debug options",	0},
 	{"destructive",		'n',	NULL,		0,
 		"Do not restore blocks of the device after probing it",	2},
+	{"min-memory",		'l',	NULL,		0,
+		"Trade speed for less use of memory",		0},
 	{"manual-reset",	'm',	NULL,		0,
 		"Ask user to manually reset the drive",	0},
 	{ 0 }
@@ -52,18 +54,21 @@ struct args {
 	char		*filename;
 
 	/* Debugging options. */
-	bool		unit_test;
 	bool		debug;
+	bool		unit_test;
 	bool		keep_file;
-	bool		save;
 
+	/* Behavior options. */
+	bool		save;
+	bool		min_mem;
+	bool		manual_reset;
+	/* 2 free bytes. */
+
+	/* Geometry. */
 	uint64_t	real_size_byte;
 	uint64_t	fake_size_byte;
 	int		wrap;
 	int		block_order;
-
-	bool		manual_reset;
-	/* 3 free bytes. */
 };
 
 static long long arg_to_long_long(const struct argp_state *state,
@@ -137,6 +142,10 @@ static error_t parse_opt(int key, char *arg, struct argp_state *state)
 		args->save = false;
 		break;
 
+	case 'l':
+		args->min_mem = true;
+		break;
+
 	case 'm':
 		args->manual_reset = true;
 		break;
@@ -192,11 +201,14 @@ static const struct unit_test_item ftype_to_params[] = {
 	/* Geometry of a real limbo drive. */
 	{1777645568ULL,	32505331712ULL,	35,	9},
 
-	/* Geometry of a real wraparound drive. */
+	/* Wraparound drive. */
 	{1ULL << 31,	1ULL << 34,	31,	9},
 
 	/* Chain drive. */
 	{1ULL << 31,	1ULL << 34,	32,	9},
+
+	/* Extreme case for memory usage (limbo drive). */
+	{1ULL << 20,	1ULL << 40,	40,	9},
 };
 
 #define UNIT_TEST_N_CASES \
@@ -297,7 +309,17 @@ static int test_device(struct args *args)
 	assert(dev);
 
 	if (args->save) {
-		dev = create_safe_device(dev, probe_device_max_blocks(dev));
+		dev = create_safe_device(dev, probe_device_max_blocks(dev),
+			args->min_mem);
+		if (!dev) {
+			if (!args->min_mem)
+				fprintf(stderr, "Out of memory, try `f3probe --min-memory %s'\n",
+					args->filename);
+			else
+				fprintf(stderr, "Out of memory, try `f3probe --destructive %s'\nPlease back your data up before using option --destructive.\nAlternatively, you could use a machine with more memory to run f3probe.\n",
+					args->filename);
+			exit(1);
+		}
 		assert(dev);
 	}
 
@@ -346,15 +368,16 @@ int main(int argc, char **argv)
 {
 	struct args args = {
 		/* Defaults. */
-		.unit_test	= false,
 		.debug		= false,
+		.unit_test	= false,
 		.keep_file	= false,
 		.save		= true,
+		.min_mem	= false,
+		.manual_reset	= false,
 		.real_size_byte	= 1ULL << 31,
 		.fake_size_byte	= 1ULL << 34,
 		.wrap		= 31,
 		.block_order	= 9,
-		.manual_reset	= false,
 	};
 
 	/* Read parameters. */
diff --git a/libprobe.c b/libprobe.c
index db5a777..7cba2af 100644
--- a/libprobe.c
+++ b/libprobe.c
@@ -570,33 +570,59 @@ static inline void *align_512(void *p)
 	return (void *)(   (ip + 511) & ~511   );
 }
 
-static int sdev_write_block(struct device *dev, const char *buf, int length,
-	uint64_t offset)
+static int sdev_save_block(struct safe_device *sdev,
+	int length, uint64_t offset)
 {
-	struct safe_device *sdev = dev_sdev(dev);
 	const int block_order = dev_get_block_order(sdev->shadow_dev);
 	lldiv_t idx = lldiv(offset >> block_order, SDEV_BITMAP_BITS_PER_WORD);
 	SDEV_BITMAP_WORD set_bit = (SDEV_BITMAP_WORD)1 << idx.rem;
+	char *block;
+	int rc;
 
 	/* The current implementation doesn't support variable lengths. */
 	assert(length == dev_get_block_size(sdev->shadow_dev));
 
 	/* Is this block already saved? */
-	if (!(sdev->sb_bitmap[idx.quot] & set_bit)) {
-		/* No. Save this block. */
-		char *block = (char *)align_512(sdev->saved_blocks) +
-			(sdev->sb_n << block_order);
-		int rc;
-		assert(sdev->sb_n < sdev->sb_max);
-		rc = sdev->shadow_dev->read_block(sdev->shadow_dev, block,
-			length, offset);
-		if (rc)
-			return rc;
-		sdev->sb_bitmap[idx.quot] |= set_bit;
-		sdev->sb_offsets[sdev->sb_n] = offset;
-		sdev->sb_n++;
+	if (!sdev->sb_bitmap) {
+		int i;
+		/* Running without bitmap. */
+		for (i = 0; i < sdev->sb_n; i++)
+			if (sdev->sb_offsets[i] == offset) {
+				/* The block at @offset is already saved. */
+				return 0;
+			}
+	} else if (sdev->sb_bitmap[idx.quot] & set_bit) {
+		/* The block at @offset is already saved. */
+		return 0;
 	}
 
+	/* The block at @offset hasn't been saved before. Save this block. */
+	assert(sdev->sb_n < sdev->sb_max);
+	block = (char *)align_512(sdev->saved_blocks) +
+		(sdev->sb_n << block_order);
+	rc = sdev->shadow_dev->read_block(sdev->shadow_dev, block,
+		length, offset);
+	if (rc)
+		return rc;
+
+	/* Bookkeeping. */
+	if (sdev->sb_bitmap)
+		sdev->sb_bitmap[idx.quot] |= set_bit;
+	sdev->sb_offsets[sdev->sb_n] = offset;
+	sdev->sb_n++;
+	return 0;
+}
+
+static int sdev_write_block(struct device *dev, const char *buf, int length,
+	uint64_t offset)
+{
+	struct safe_device *sdev = dev_sdev(dev);
+	int rc;
+
+	rc = sdev_save_block(sdev, length, offset);
+	if (rc)
+		return rc;
+
 	return sdev->shadow_dev->write_block(sdev->shadow_dev, buf,
 		length, offset);
 }
@@ -638,12 +664,11 @@ static void sdev_free(struct device *dev)
 	free_device(sdev->shadow_dev);
 }
 
-struct device *create_safe_device(struct device *dev, int max_blocks)
+struct device *create_safe_device(struct device *dev, int max_blocks,
+	int min_memory)
 {
 	struct safe_device *sdev;
 	const int block_order = dev_get_block_order(dev);
-	lldiv_t idx = lldiv(dev_get_size_byte(dev) >> block_order,
-		SDEV_BITMAP_BITS_PER_WORD);
 	uint64_t length;
 
 	sdev = malloc(sizeof(*sdev));
@@ -659,11 +684,18 @@ struct device *create_safe_device(struct device *dev, int max_blocks)
 	if (!sdev->sb_offsets)
 		goto saved_blocks;
 
-	length = (idx.quot + (idx.rem ? 1 : 0)) * sizeof(SDEV_BITMAP_WORD);
-	sdev->sb_bitmap = malloc(length);
-	if (!sdev->sb_bitmap)
-		goto offsets;
-	memset(sdev->sb_bitmap, 0, length);
+	if (!min_memory) {
+		lldiv_t idx = lldiv(dev_get_size_byte(dev) >> block_order,
+			SDEV_BITMAP_BITS_PER_WORD);
+		length = (idx.quot + (idx.rem ? 1 : 0)) *
+			sizeof(SDEV_BITMAP_WORD);
+		sdev->sb_bitmap = malloc(length);
+		if (!sdev->sb_bitmap)
+			goto offsets;
+		memset(sdev->sb_bitmap, 0, length);
+	} else {
+		sdev->sb_bitmap = NULL;
+	}
 
 	sdev->shadow_dev = dev;
 	sdev->sb_n = 0;
diff --git a/libprobe.h b/libprobe.h
index 074f450..b524a5a 100644
--- a/libprobe.h
+++ b/libprobe.h
@@ -38,7 +38,8 @@ struct device *create_file_device(const char *filename,
 
 struct device *create_block_device(const char *filename, int manual_reset);
 
-struct device *create_safe_device(struct device *dev, int max_blocks);
+struct device *create_safe_device(struct device *dev, int max_blocks,
+	int min_memory);
 
 void free_device(struct device *dev);