--- vcd.c.orig
+++ vcd.c
@@ -161,9 +161,27 @@
 			n = pwrite(sc->fd, bp->b_data,
 				   bp->b_bcount, bio->bio_offset);
 			break;
+		case BUF_CMD_FLUSH:
+			/*
+			 * Mirror vkd_doio() (vdisk.c:332-337): push any
+			 * cached writes in the backing file to the store.
+			 * HAMMER issues device flushes during mount
+			 * recovery and on every flusher cycle
+			 * (hammer_io_flush_sync, hammer_io.c:1985-2007).
+			 */
+			if (fsync(sc->fd) < 0)
+				n = -1;
+			else
+				n = bp->b_bcount;
+			break;
 		default:
-			panic("vcd: bad b_cmd %d", bp->b_cmd);
-			break; /* not reached */
+			/*
+			 * Other commands (e.g. BUF_CMD_FREEBLKS/TRIM):
+			 * nothing to do for a file-backed virtual CD.
+			 * Complete the bio instead of panicking.
+			 */
+			n = bp->b_bcount;
+			break;
 		}
 		if (n != bp->b_bcount) {
 			bp->b_error = EIO;