diff --git a/cmdline/elem.c b/cmdline/elem.c
index ebf3e569..3ec737f7 100644
--- a/cmdline/elem.c
+++ b/cmdline/elem.c
@@ -1389,6 +1389,20 @@ static int extent_parity_inside_compare_unlock(const void* void_a, const void* v
 	return 0;
 }
 
+struct snapraid_extent* fs_par2extent_find_next(struct snapraid_disk* disk, block_off_t parity_pos)
+{
+	struct extent_parity_inside arg = { parity_pos };
+	struct snapraid_extent* extent;
+
+	fs_lock(disk);
+
+	/* non-overlapping extents let this search find the containing extent or the next one */
+	extent = tommy_tree_search_greater_equal_compare(&disk->fs_parity, extent_parity_inside_compare_unlock, &arg);
+
+	fs_unlock(disk);
+	return extent;
+}
+
 /**
  * Search the extent at the specified parity position.
  * The search is optimized for sequential accesses.
diff --git a/cmdline/elem.h b/cmdline/elem.h
index e767b024..b9a638ae 100644
--- a/cmdline/elem.h
+++ b/cmdline/elem.h
@@ -1243,6 +1243,13 @@ void fs_deallocate(struct snapraid_disk* disk, block_off_t pos);
  */
 struct snapraid_block* fs_file2block_get(struct snapraid_file* file, block_off_t file_pos);
 
+/**
+ * Find the first extent ending after the specified parity position.
+ * Return 0 if no such extent exists. The caller must ensure that the extent
+ * trees remain unchanged while using the returned extent or its tree node.
+ */
+struct snapraid_extent* fs_par2extent_find_next(struct snapraid_disk* disk, block_off_t parity_pos);
+
 /**
  * Get the file position from the parity position.
  * Return 0 if no file is using it.
diff --git a/cmdline/selftest.c b/cmdline/selftest.c
index 0072b318..602f80e3 100644
--- a/cmdline/selftest.c
+++ b/cmdline/selftest.c
@@ -2270,7 +2270,7 @@ static void test_unsynced_sweep(void)
 	state.block_size = 4096;
 	tommy_list_init(&state.disklist);
 
-	if (fs_count_unsynced(&state, 100) != 0) {
+	if (fs_count_unsynced(&state, 0, 100) != 0) {
 		log_fatal(EINTERNAL, "test_unsynced_sweep: no disks failed\n");
 		exit(EXIT_FAILURE);
 	}
@@ -2281,7 +2281,7 @@ static void test_unsynced_sweep(void)
 	tommy_list_insert_tail(&state.disklist, &disk2->node, disk2);
 
 	/* initially both disks are empty */
-	if (fs_count_unsynced(&state, 100) != 0) {
+	if (fs_count_unsynced(&state, 10, 100) != 0) {
 		log_fatal(EINTERNAL, "test_unsynced_sweep: empty disks failed\n");
 		exit(EXIT_FAILURE);
 	}
@@ -2301,7 +2301,7 @@ static void test_unsynced_sweep(void)
 	fs_allocate(disk2, 0, file2, 0, 20);
 
 	/* parity 0..14 has only BLK or EMPTY; parity 15..19 has CHG on disk1 and BLK on disk2 -> 5 unsynced */
-	if (fs_count_unsynced(&state, 20) != 5) {
+	if (fs_count_unsynced(&state, 15, 20) != 5) {
 		log_fatal(EINTERNAL, "test_unsynced_sweep: mixed block run failed\n");
 		exit(EXIT_FAILURE);
 	}
@@ -2313,7 +2313,7 @@ static void test_unsynced_sweep(void)
 	fs_allocate(disk1, 20, del, 0, 3);
 
 	/* disk2 is empty at 20..22 -> DELETED alone does not count as unsynced */
-	if (fs_count_unsynced(&state, 23) != 5) {
+	if (fs_count_unsynced(&state, 15, 23) != 5) {
 		log_fatal(EINTERNAL, "test_unsynced_sweep: deleted without other file failed\n");
 		exit(EXIT_FAILURE);
 	}
@@ -2330,7 +2330,7 @@ static void test_unsynced_sweep(void)
 
 	/* parity 20: DELETED on d1, empty on d2 -> not unsynced;
 	 * parity 21..22: DELETED on d1, BLK on d2 -> unsynced (2 blocks). Total 5 + 2 = 7 */
-	if (fs_count_unsynced(&state, 23) != 7) {
+	if (fs_count_unsynced(&state, 15, 23) != 7) {
 		log_fatal(EINTERNAL, "test_unsynced_sweep: deleted with file failed\n");
 		exit(EXIT_FAILURE);
 	}
@@ -2349,14 +2349,19 @@ static void test_unsynced_sweep(void)
 	 * Positions 30..33 are all unsynced: REP, REP, CHG, REBUILD respectively.
 	 * At 32 and 33 the invalid block changes disks at the same event boundary;
 	 * count each position once using the refreshed flags from both disks.
-	 * Check limits in gaps, within extents and runs, and at their boundaries.
+	 * Check every window between limits in gaps, within extents and runs,
+	 * and at their boundaries, including empty windows and nonzero starts.
 	 */
 	for (i = 0; i < sizeof(bounds) / sizeof(bounds[0]); ++i) {
-		uint64_t count = fs_count_unsynced(&state, bounds[i].blockmax);
-		if (count != bounds[i].unsynced) {
-			log_fatal(EINTERNAL, "test_unsynced_sweep: limit %" PRIu64 " expected %" PRIu64 ", got %" PRIu64 "\n",
-				bounds[i].blockmax, bounds[i].unsynced, count);
-			exit(EXIT_FAILURE);
+		block_off_t j;
+		for (j = 0; j <= i; ++j) {
+			uint64_t expected = bounds[i].unsynced - bounds[j].unsynced;
+			uint64_t count = fs_count_unsynced(&state, bounds[j].blockmax, bounds[i].blockmax);
+			if (count != expected) {
+				log_fatal(EINTERNAL, "test_unsynced_sweep: window [%" PRIu64 ", %" PRIu64 ") expected %" PRIu64 ", got %" PRIu64 "\n",
+					bounds[j].blockmax, bounds[i].blockmax, expected, count);
+				exit(EXIT_FAILURE);
+			}
 		}
 	}
 
diff --git a/cmdline/state.c b/cmdline/state.c
index 81a41914..ce9afc4d 100644
--- a/cmdline/state.c
+++ b/cmdline/state.c
@@ -2242,6 +2242,10 @@ static inline void disk_advance(
 		block_off_t run_rem = 1;
 		const unsigned char* ptr = (const unsigned char*)block;
 
+		/* stop inspecting block flags at the end of the requested window */
+		if (extent_rem > blockmax - pos)
+			extent_rem = blockmax - pos;
+
 		*has_file_ptr = has_file;
 		*has_invalid_ptr = has_invalid;
 
@@ -2264,14 +2268,16 @@ static inline void disk_advance(
  * block, and at least one disk has an invalid parity block (CHG, REP, REBUILD,
  * or DELETED).
  *
- * The sweep takes O(B + R * D) time, where B is the number of blocks inspected,
+ * Counts only positions in [blockmin, blockmax).
+ * The sweep takes O(D * log(E) + B + R * D) time, where E is the number of
+ * extents per disk, B is the number of blocks inspected within the window,
  * R is the number of event positions processed (including initialization), and
  * D is the number of disks. Cached event boundaries avoid rescanning block runs.
  *
  * The caller must ensure that the disk list, extent trees, and block states
- * remain unchanged during the count, as this function does not acquire locks.
+ * remain unchanged during the count, as no locks are held during the sweep.
  */
-uint64_t fs_count_unsynced(struct snapraid_state* state, block_off_t blockmax)
+uint64_t fs_count_unsynced(struct snapraid_state* state, block_off_t blockmin, block_off_t blockmax)
 {
 	tommy_tree_node* cursors[RAID_DATA_MAX];
 	block_off_t next_events[RAID_DATA_MAX];
@@ -2280,7 +2286,7 @@ uint64_t fs_count_unsynced(struct snapraid_state* state, block_off_t blockmax)
 	unsigned disk_count = 0;
 	tommy_node* node;
 	uint64_t total_unsynced = 0;
-	block_off_t pos = 0;
+	block_off_t pos = blockmin;
 	size_t stride = block_sizeof();
 	unsigned d;
 
@@ -2294,9 +2300,10 @@ uint64_t fs_count_unsynced(struct snapraid_state* state, block_off_t blockmax)
 
 	for (node = state->disklist; node != 0; node = node->next) {
 		struct snapraid_disk* disk = node->data;
-		cursors[disk_count] = tommy_tree_head(&disk->fs_parity);
+		struct snapraid_extent* ext = fs_par2extent_find_next(disk, blockmin);
+		cursors[disk_count] = ext ? &ext->parity_node : 0;
 		/* force an initial event so flags are set before their first use */
-		next_events[disk_count] = 0;
+		next_events[disk_count] = blockmin;
 		++disk_count;
 	}
 
@@ -2508,6 +2515,8 @@ struct state_read_context {
 static void state_read_content(struct snapraid_state* state, const char* path, STREAM* f, struct state_read_context* context)
 {
 	block_off_t blockmax;
+	block_off_t inv_min;
+	block_off_t inv_max;
 	uint64_t count_file;
 	uint64_t count_hardlink;
 	uint64_t count_symlink;
@@ -2523,7 +2532,6 @@ static void state_read_content(struct snapraid_state* state, const char* path, S
 	int has_hash;
 	int has_prevhash;
 	int has_info;
-	int has_invalid_parity;
 	char buffer[PATH_MAX];
 	ssize_t ret;
 	tommy_array disk_mapping;
@@ -2532,6 +2540,8 @@ static void state_read_content(struct snapraid_state* state, const char* path, S
 	tommy_hashdyn bucket_hash;
 
 	blockmax = 0;
+	inv_min = BLOCK_MAX;
+	inv_max = 0;
 	count_file = 0;
 	count_hardlink = 0;
 	count_symlink = 0;
@@ -2547,7 +2557,6 @@ static void state_read_content(struct snapraid_state* state, const char* path, S
 	has_hash = 0;
 	has_prevhash = 0;
 	has_info = 0;
-	has_invalid_parity = 0;
 	mapping_max = 0;
 	memset(disk_file_count, 0, sizeof(disk_file_count));
 	tommy_array_init(&disk_mapping);
@@ -2819,8 +2828,12 @@ static void state_read_content(struct snapraid_state* state, const char* path, S
 				}
 
 				/* all file block states except BLK require a parity update */
-				if (v_state != BLOCK_STATE_BLK)
-					has_invalid_parity = 1;
+				if (v_state != BLOCK_STATE_BLK) {
+					if (v_pos < inv_min)
+						inv_min = v_pos;
+					if (v_pos + v_count > inv_max)
+						inv_max = v_pos + v_count;
+				}
 
 				v_file_pos = v_idx;
 				ret = state_read_block_run(f, file, v_file_pos, v_count, v_state, discard_hash);
@@ -2989,14 +3002,6 @@ static void state_read_content(struct snapraid_state* state, const char* path, S
 					}
 				}
 			}
-
-			/*
-			 * A synchronized content has no invalid block state, so its unsynced
-			 * count is certainly zero. Scan the extent maps only when needed.
-			 */
-			if (has_invalid_parity) {
-				count_unsynced = fs_count_unsynced(state, blockmax);
-			}
 		} else if (c == 'h') {
 			/* hole */
 			block_off_t v_pos;
@@ -3049,7 +3054,10 @@ static void state_read_content(struct snapraid_state* state, const char* path, S
 				switch (c) {
 				case 'o' :
 					/* if it's a run of deleted blocks */
-					has_invalid_parity = 1;
+					if (v_pos < inv_min)
+						inv_min = v_pos;
+					if (v_pos + v_count > inv_max)
+						inv_max = v_pos + v_count;
 
 					/* ensure the converted byte size fits in signed data_off_t without overflow */
 					if (v_count > INT64_MAX / (uint64_t)state->block_size) {
@@ -4023,6 +4031,16 @@ static void state_read_content(struct snapraid_state* state, const char* path, S
 		/* LCOV_EXCL_STOP */
 	}
 
+	/*
+	 * Every invalid file or deleted run expands this window while parsing.
+	 * Outside it all parity is valid, so no position can be unsynced.
+	 * Count only after all records are loaded, independently of the info record.
+	 * A synchronized content leaves the window empty and needs no sweep.
+	 */
+	if (inv_min < inv_max) {
+		count_unsynced = fs_count_unsynced(state, inv_min, inv_max);
+	}
+
 	context->blockmax = blockmax;
 	context->count_map = mapping_max;
 	context->count_file = count_file;
diff --git a/cmdline/state.h b/cmdline/state.h
index 441e1415..68ee7515 100644
--- a/cmdline/state.h
+++ b/cmdline/state.h
@@ -242,10 +242,10 @@ void state_read(struct snapraid_state* state);
 
 /**
  * Count the number of unsynced parity positions using an event sweep across extents.
- * Counts positions in [0, blockmax). The caller must ensure that the disk list,
+ * Counts positions in [blockmin, blockmax). The caller must ensure that the disk list,
  * extent trees, and block states remain unchanged for the duration of the call.
  */
-uint64_t fs_count_unsynced(struct snapraid_state* state, block_off_t blockmax);
+uint64_t fs_count_unsynced(struct snapraid_state* state, block_off_t blockmin, block_off_t blockmax);
 
 /**
  * Write the new state.