diff --git a/mars.h b/mars.h
index bc86d93e..43775588 100644
--- a/mars.h
+++ b/mars.h
@@ -346,6 +346,7 @@ static inline void unuse_fake_mm(void) {}
 
 extern int mars_digest_size;
 extern void mars_digest(unsigned char *digest, void *data, int len);
+extern void mref_checksum(struct mref_object *mref);
 
 /////////////////////////////////////////////////////////////////////////
 
diff --git a/mars_aio.c b/mars_aio.c
index e121cced..2f6e87a8 100644
--- a/mars_aio.c
+++ b/mars_aio.c
@@ -208,6 +208,7 @@ void _complete(struct aio_output *output, struct mref_object *mref, int err)
 	if (err < 0) {
 		MARS_ERR("IO error %d at pos=%lld len=%d (mref=%p ref_data=%p)\n", err, mref->ref_pos, mref->ref_len, mref, mref->ref_data);
 	} else {
+		mref_checksum(mref);
 		mref->ref_flags |= MREF_UPTODATE;
 	}
 
diff --git a/mars_bio.c b/mars_bio.c
index 3f359ef5..d668ecf5 100644
--- a/mars_bio.c
+++ b/mars_bio.c
@@ -441,6 +441,7 @@ static int bio_thread(void *data)
 			if (code < 0) {
 				MARS_ERR("IO error %d\n", code);
 			} else {
+				mref_checksum(mref);
 				mref->ref_flags |= MREF_UPTODATE;
 			}
 
diff --git a/mars_client.c b/mars_client.c
index 0c797d88..6a94e999 100644
--- a/mars_client.c
+++ b/mars_client.c
@@ -262,7 +262,7 @@ int receiver_thread(void *data)
 		if (status < 0)
 			goto done;
 
-		switch (cmd.cmd_code) {
+		switch (cmd.cmd_code & CMD_FLAG_MASK) {
 		case CMD_NOTIFY:
 			mars_trigger();
 			break;
@@ -303,7 +303,7 @@ int receiver_thread(void *data)
 
 			MARS_IO("got callback id = %d, old pos = %lld len = %d rw = %d\n", mref->ref_id, mref->ref_pos, mref->ref_len, mref->ref_rw);
 
-			status = mars_recv_cb(&output->socket, mref);
+			status = mars_recv_cb(&output->socket, mref, &cmd);
 			MARS_IO("new status = %d, pos = %lld len = %d rw = %d\n", status, mref->ref_pos, mref->ref_len, mref->ref_rw);
 			if (status < 0) {
 				MARS_WRN("interrupted data transfer during callback, status = %d\n", status);
diff --git a/mars_generic.c b/mars_generic.c
index 0f268351..a0c73e9b 100644
--- a/mars_generic.c
+++ b/mars_generic.c
@@ -123,6 +123,23 @@ void mars_digest(unsigned char *digest, void *data, int len)
 }
 EXPORT_SYMBOL_GPL(mars_digest);
 
+void mref_checksum(struct mref_object *mref)
+{
+	unsigned char checksum[mars_digest_size];
+	int len;
+
+	if (mref->ref_cs_mode <= 0 || !mref->ref_data)
+		return;
+
+	mars_digest(checksum, mref->ref_data, mref->ref_len);
+
+	len = sizeof(mref->ref_checksum);
+	if (len > mars_digest_size)
+		len = mars_digest_size;
+	memcpy(&mref->ref_checksum, checksum, len);
+}
+EXPORT_SYMBOL_GPL(mref_checksum);
+
 /////////////////////////////////////////////////////////////////////
 
 // tracing
diff --git a/mars_net.c b/mars_net.c
index be60d579..e0bdc959 100644
--- a/mars_net.c
+++ b/mars_net.c
@@ -762,16 +762,19 @@ int mars_send_mref(struct mars_socket *msock, struct mref_object *mref)
 	int seq = 0;
 	int status;
 
+	if (mref->ref_rw != 0 && mref->ref_data && mref->ref_cs_mode < 2)
+		cmd.cmd_code |= CMD_FLAG_HAS_DATA;
+
 	status = _mars_send_struct(msock, &cmd, mars_cmd_meta, &seq, true);
 	if (status < 0)
 		goto done;
 
 	seq = 0;
-	status = _mars_send_struct(msock, mref, mars_mref_meta, &seq, mref->ref_rw != 0);
+	status = _mars_send_struct(msock, mref, mars_mref_meta, &seq, cmd.cmd_code & CMD_FLAG_HAS_DATA);
 	if (status < 0)
 		goto done;
 
-	if (mref->ref_rw != 0) {
+	if (cmd.cmd_code & CMD_FLAG_HAS_DATA) {
 		status = mars_send_raw(msock, mref->ref_data, mref->ref_len, false);
 	}
 done:
@@ -779,7 +782,7 @@ done:
 }
 EXPORT_SYMBOL_GPL(mars_send_mref);
 
-int mars_recv_mref(struct mars_socket *msock, struct mref_object *mref)
+int mars_recv_mref(struct mars_socket *msock, struct mref_object *mref, struct mars_cmd *cmd)
 {
 	int seq = 0;
 	int status;
@@ -787,7 +790,8 @@ int mars_recv_mref(struct mars_socket *msock, struct mref_object *mref)
 	status = _mars_recv_struct(msock, mref, mars_mref_meta, &seq, __LINE__);
 	if (status < 0)
 		goto done;
-	if (mref->ref_rw) {
+
+	if (cmd->cmd_code & CMD_FLAG_HAS_DATA) {
 		if (!mref->ref_data)
 			mref->ref_data = brick_zmem_alloc(mref->ref_len);
 		if (!mref->ref_data) {
@@ -812,16 +816,19 @@ int mars_send_cb(struct mars_socket *msock, struct mref_object *mref)
 	int seq = 0;
 	int status;
 
+	if (mref->ref_rw == 0 && mref->ref_data && mref->ref_cs_mode < 2)
+		cmd.cmd_code |= CMD_FLAG_HAS_DATA;
+
 	status = _mars_send_struct(msock, &cmd, mars_cmd_meta, &seq, true);
 	if (status < 0)
 		goto done;
 
 	seq = 0;
-	status = _mars_send_struct(msock, mref, mars_mref_meta, &seq, !mref->ref_rw);
+	status = _mars_send_struct(msock, mref, mars_mref_meta, &seq, cmd.cmd_code & CMD_FLAG_HAS_DATA);
 	if (status < 0)
 		goto done;
 
-	if (!mref->ref_rw) {
+	if (cmd.cmd_code & CMD_FLAG_HAS_DATA) {
 		MARS_IO("#%d sending blocklen = %d\n", msock->s_debug_nr, mref->ref_len);
 		status = mars_send_raw(msock, mref->ref_data, mref->ref_len, false);
 	}
@@ -830,7 +837,7 @@ done:
 }
 EXPORT_SYMBOL_GPL(mars_send_cb);
 
-int mars_recv_cb(struct mars_socket *msock, struct mref_object *mref)
+int mars_recv_cb(struct mars_socket *msock, struct mref_object *mref, struct mars_cmd *cmd)
 {
 	int seq = 0;
 	int status;
@@ -838,7 +845,8 @@ int mars_recv_cb(struct mars_socket *msock, struct mref_object *mref)
 	status = _mars_recv_struct(msock, mref, mars_mref_meta, &seq, __LINE__);
 	if (status < 0)
 		goto done;
-	if (!mref->ref_rw) {
+
+	if (cmd->cmd_code & CMD_FLAG_HAS_DATA) {
 		if (!mref->ref_data) {
 			MARS_WRN("#%d no internal buffer available\n", msock->s_debug_nr);
 			status = -EINVAL;
diff --git a/mars_net.h b/mars_net.h
index 3dd4d4da..943417dd 100644
--- a/mars_net.h
+++ b/mars_net.h
@@ -43,6 +43,9 @@ enum {
 	CMD_CB,
 };
 
+#define CMD_FLAG_MASK     255
+#define CMD_FLAG_HAS_DATA 256
+
 struct mars_cmd {
 	struct timespec cmd_stamp; // for automatic lamport clock
 	int cmd_code;
@@ -88,9 +91,9 @@ extern int mars_send_dent_list(struct mars_socket *msock, struct list_head *anch
 extern int mars_recv_dent_list(struct mars_socket *msock, struct list_head *anchor);
 
 extern int mars_send_mref(struct mars_socket *msock, struct mref_object *mref);
-extern int mars_recv_mref(struct mars_socket *msock, struct mref_object *mref);
+extern int mars_recv_mref(struct mars_socket *msock, struct mref_object *mref, struct mars_cmd *cmd);
 extern int mars_send_cb(struct mars_socket *msock, struct mref_object *mref);
-extern int mars_recv_cb(struct mars_socket *msock, struct mref_object *mref);
+extern int mars_recv_cb(struct mars_socket *msock, struct mref_object *mref, struct mars_cmd *cmd);
 
 /////////////////////////////////////////////////////////////////////////
 
diff --git a/mars_server.c b/mars_server.c
index 1a5dc9d9..fc312e97 100644
--- a/mars_server.c
+++ b/mars_server.c
@@ -139,7 +139,7 @@ err:
 	MARS_FAT("cannot handle callback - giving up\n");
 }
 
-int server_io(struct server_brick *brick, struct mars_socket *sock)
+int server_io(struct server_brick *brick, struct mars_socket *sock, struct mars_cmd *cmd)
 {
 	struct mref_object *mref;
 	struct server_mref_aspect *mref_a;
@@ -159,7 +159,7 @@ int server_io(struct server_brick *brick, struct mars_socket *sock)
 		goto done;
 	}
 
-	status = mars_recv_mref(sock, mref);
+	status = mars_recv_mref(sock, mref, cmd);
 	if (status < 0) {
 		mars_free_mref(mref);
 		goto done;
@@ -299,7 +299,7 @@ int handler_thread(void *data)
 		MARS_IO("cmd = %d\n", cmd.cmd_code);
 
 		status = -EPROTO;
-		switch (cmd.cmd_code) {
+		switch (cmd.cmd_code & CMD_FLAG_MASK) {
 		case CMD_NOP:
 			MARS_DBG("got NOP operation\n");
 			status = 0;
@@ -393,7 +393,7 @@ int handler_thread(void *data)
 				break;
 			}
 #endif
-			status = server_io(brick, sock);
+			status = server_io(brick, sock, &cmd);
 			break;
 		}
 		case CMD_CB:
diff --git a/mars_sio.c b/mars_sio.c
index fa994897..de3ff6f0 100644
--- a/mars_sio.c
+++ b/mars_sio.c
@@ -324,6 +324,7 @@ void _complete(struct sio_output *output, struct mref_object *mref, int err)
 	if (err < 0) {
 		MARS_ERR("IO error %d at pos=%lld len=%d (mref=%p ref_data=%p)\n", err, mref->ref_pos, mref->ref_len, mref, mref->ref_data);
 	} else {
+		mref_checksum(mref);
 		mref->ref_flags |= MREF_UPTODATE;
 	}
 
diff --git a/mars_trans_logger.c b/mars_trans_logger.c
index c09e2aae..7b13eed7 100644
--- a/mars_trans_logger.c
+++ b/mars_trans_logger.c
@@ -1264,6 +1264,7 @@ void _complete(struct trans_logger_brick *brick, struct trans_logger_mref_aspect
 
 	orig_mref_a->is_completed = true;
 	if (likely(error >= 0)) {
+		mref_checksum(orig_mref);
 		orig_mref->ref_flags &= ~MREF_WRITING;
 		orig_mref->ref_flags |= MREF_UPTODATE;
 	}