diff --git a/kernel/mars_copy.c b/kernel/mars_copy.c
index e01c08b8..db4e1db8 100644
--- a/kernel/mars_copy.c
+++ b/kernel/mars_copy.c
@@ -230,27 +230,19 @@ void copy_endio(struct generic_callback *cb)
 		error = -EINVAL;
 		goto exit;
 	}
-	st->active[queue] = false;
-	if (unlikely(st->table[queue])) {
+	if (unlikely(st->table[queue] != mref)) {
 		MARS_ERR("table corruption at %d %d (%p => %p)\n", index, queue, st->table[queue], mref);
 		error = -EEXIST;
 		goto exit;
 	}
 	if (unlikely(cb->cb_error < 0)) {
 		error = cb->cb_error;
-		__clear_mref(brick, mref, queue);
 		/* This is racy, but does no harm.
 		 * Worst case just produces more error output.
 		 */
 		if (!brick->copy_error_count++) {
 			MARS_WRN("IO error %d on index %d, old state = %d\n", cb->cb_error, index, st->state);
 		}
-	} else {
-		if (unlikely(st->table[queue])) {
-			MARS_ERR("overwriting index %d, state = %d\n", index, st->state);
-			_clear_mref(brick, index, queue);
-		}
-		st->table[queue] = mref;
 	}
 
 exit:
@@ -258,6 +250,7 @@ exit:
 		st->error = error;
 		_clash(brick);
 	}
+	st->active[queue] = false;
 	if (mref->ref_rw) {
 		atomic_dec(&brick->copy_write_flight);
 	} else {
@@ -277,6 +270,7 @@ int _make_mref(struct copy_brick *brick, int index, int queue, void *data, loff_
 	struct mref_object *mref;
 	struct copy_mref_aspect *mref_a;
 	struct copy_input *input;
+	struct copy_state *st;
 	int offset;
 	int len;
 	int status = -EAGAIN;
@@ -335,7 +329,10 @@ int _make_mref(struct copy_brick *brick, int index, int queue, void *data, loff_
 
 	//MARS_IO("queue = %d index = %d pos = %lld len = %d rw = %d\n", queue, index, mref->ref_pos, mref->ref_len, rw);
 
-	GET_STATE(brick, index).active[queue] = true;
+	st = &GET_STATE(brick, index);
+	st->table[queue] = mref;
+	st->active[queue] = true;
+
 	if (rw) {
 		atomic_inc(&brick->copy_write_flight);
 	} else {
@@ -422,8 +419,6 @@ restart:
 			goto idle;
 		}
 
-		_clear_mref(brick, index, 1);
-		_clear_mref(brick, index, 0);
 		st->writeout = false;
 		st->error = 0;
 
@@ -455,17 +450,16 @@ restart:
 		next_state = COPY_STATE_READ2;
 		/* fallthrough */
 	case COPY_STATE_READ2:
-		mref1 = st->table[1];
-		if (!mref1) { // idempotence: wait by unchanged state
+		if (st->active[1]) { // idempotence: wait by unchanged state
 			goto idle;
 		}
 		/* fallthrough => wait for both mrefs to appear */
 	case COPY_STATE_READ1:
 	case COPY_STATE_READ3:
-		mref0 = st->table[0];
-		if (!mref0) { // idempotence: wait by unchanged state
+		if (st->active[0]) { // idempotence: wait by unchanged state
 			goto idle;
 		}
+		mref0 = st->table[0];
 		if (brick->copy_limiter) {
 			int amount = (mref0->ref_len - 1) / 1024 + 1;
 			mars_limit_sleep(brick->copy_limiter, amount);
@@ -543,6 +537,11 @@ restart:
 			progress = -EILSEQ;
 			break;
 		}
+		if (unlikely(st->active[0])) {
+			MARS_ERR("src buffer for write is active, state %d at index %d\n", state, index);
+			progress = -EILSEQ;
+			break;
+		}
 		if (unlikely(brick->is_aborting)) {
 			progress = -EINTR;
 			break;
@@ -565,8 +564,7 @@ restart:
 		next_state = COPY_STATE_WRITTEN;
 		/* fallthrough */
 	case COPY_STATE_WRITTEN:
-		mref1 = st->table[1];
-		if (!mref1) { // idempotence: wait by unchanged state
+		if (st->active[1]) { // idempotence: wait by unchanged state
 			MARS_IO("irrelevant\n");
 			goto idle;
 		}