diff --git a/src/pv/transfer.c b/src/pv/transfer.c
index f487335..d2af274 100644
--- a/src/pv/transfer.c
+++ b/src/pv/transfer.c
@@ -710,6 +710,19 @@ static int pv__transfer_write(pvstate_t state, bool *eof_in, bool *eof_out, long
 					debug("%s: %s", "output lseek() failed", strerror(errno));
 					state->transfer.output_not_seekable = true;
 				} else if (0 != ftruncate(state->control.output_fd, output_offset)) {
+					/* TODO: don't ftruncate() immediately. */
+					/*
+					 * On XFS filesystems it looks like
+					 * this can actually make a sparse
+					 * file take up more space than a
+					 * regular file, though it's not
+					 * obvious yet whether the
+					 * ftruncate() is causing it or just
+					 * the lseek().
+					 *
+					 * Maybe just ftruncate() at the end
+					 * of the program.
+					 */
 					debug("%s: %s", "output ftruncate() failed", strerror(errno));
 					state->transfer.output_not_seekable = true;
 				} else {