diff --git a/cmdline/io.c b/cmdline/io.c
index aae66dc6..4064503f 100644
--- a/cmdline/io.c
+++ b/cmdline/io.c
@@ -489,7 +489,7 @@ static void io_write_next_outside(struct snapraid_io* io)
  *
  * It must be called inside the lock.
  */
-static void io_write_next_inside(struct snapraid_io* io, unsigned blockcur, int skip, int* writer_error)
+static void io_write_next_inside(struct snapraid_io* io, block_off_t blockcur, int skip, int* writer_error)
 {
 	unsigned i;
 
@@ -537,6 +537,38 @@ void io_write_next(struct snapraid_io* io, unsigned blockcur, int skip, int* wri
 #endif
 }
 
+block_off_t io_write_and_read_next(struct snapraid_io* io, block_off_t blockcur, int skip, int* writer_error, void*** buffer)
+{
+	block_off_t blockcur_schedule;
+	block_off_t blockcur_caller;
+
+	io_write_next_outside(io);
+	blockcur_schedule = io_read_next_outside(io);
+
+	/* the synchronization is protected by the io mutex */
+	pthread_mutex_lock(&io->mutex);
+
+	io_write_next_inside(io, blockcur, skip, writer_error);
+
+	blockcur_caller = io_read_next_inside(io, blockcur_schedule, buffer);
+
+#ifndef CHECKER
+	/* without the thread checker unlock before signaling, */
+	/* this reduces the number of context switches */
+	pthread_mutex_unlock(&io->mutex);
+#endif
+
+	/* signal all the workers that there is a new pending task */
+	pthread_cond_broadcast(&io->write_sched);
+	pthread_cond_broadcast(&io->read_sched);
+
+#ifdef CHECKER
+	pthread_mutex_unlock(&io->mutex);
+#endif
+
+	return blockcur_caller;
+}
+
 void io_refresh(struct snapraid_io* io)
 {
 	unsigned i;
diff --git a/cmdline/io.h b/cmdline/io.h
index 0aeadfdf..c7938860 100644
--- a/cmdline/io.h
+++ b/cmdline/io.h
@@ -365,6 +365,14 @@ void io_parity_write(struct snapraid_io* io, unsigned* levcur, unsigned* waiting
  */
 void io_write_next(struct snapraid_io* io, block_off_t blockcur, int skip, int* writer_error);
 
+/**
+ * Next write and read position.
+ *
+ * It's a combination of io_write_next() and io_read_next() that
+ * locks only one time the scheduling mutex.
+ */
+block_off_t io_write_and_read_next(struct snapraid_io* io, block_off_t blockcur, int skip, int* writer_error, void*** buffer);
+
 /**
  * Refresh the number of cached blocks for all data and parity disks.
  */
diff --git a/cmdline/sync.c b/cmdline/sync.c
index 43f474b4..ce868157 100644
--- a/cmdline/sync.c
+++ b/cmdline/sync.c
@@ -686,6 +686,8 @@ static int state_sync_process(struct snapraid_state* state, struct snapraid_pari
 	unsigned l;
 	unsigned* waiting_map;
 	unsigned waiting_mac;
+	block_off_t blocknext;
+	void** buffernext;
 
 	/* the sync process assumes that all the hashes are correct */
 	/* including the ones from CHG and DELETED blocks */
@@ -754,6 +756,10 @@ static int state_sync_process(struct snapraid_state* state, struct snapraid_pari
 	if (!state_progress_begin(state, blockstart, blockmax, countmax))
 		goto end;
 
+	/* scheduling helper */
+	buffernext = 0;
+	blocknext = 0;
+
 	while (1) {
 		unsigned failed_count;
 		int error_on_this_block;
@@ -768,7 +774,14 @@ static int state_sync_process(struct snapraid_state* state, struct snapraid_pari
 		int writer_error[IO_WRITER_ERROR_MAX];
 
 		/* go to the next block */
-		blockcur = io_read_next(&io, &buffer);
+		if (buffernext != 0) {
+			/* if already computed, use it */
+			buffer = buffernext;
+			blockcur = blocknext;
+		} else {
+			/* otherwise do it now */
+			blockcur = io_read_next(&io, &buffer);
+		}
 		if (blockcur >= blockmax)
 			break;
 
@@ -1227,7 +1240,7 @@ static int state_sync_process(struct snapraid_state* state, struct snapraid_pari
 		}
 
 		/* write finished */
-		io_write_next(&io, blockcur, !parity_going_to_be_updated, writer_error);
+		blocknext = io_write_and_read_next(&io, blockcur, !parity_going_to_be_updated, writer_error, &buffernext);
 
 		/* handle errors reported */
 		for (j = 0; j < IO_WRITER_ERROR_MAX; ++j) {