loop-util: explain why we propagate discard_max_bytes from backing block device to...

Follow-up for: 29ee9c6fb7c75c421f887c8579c65eb04d4f634d

diff --git a/src/shared/loop-util.c b/src/shared/loop-util.c
index 690f6aeb2a710f9d66e5b9f4978574a057382c20..e9064c291053ea3d52beb2400a4995c3a691a204 100644 (file)
--- a/src/shared/loop-util.c
+++ b/src/shared/loop-util.c
@@ -611,6 +611,10 @@ static int loop_device_make_internal(
         }
 
         if (S_ISBLK(st.st_mode)) {
+                /* Propagate backing device's discard byte limit to our loopback block device. We do this in
+                 * order to avoid that (supposedly quick) discard requests on the loopback device get turned
+                 * into (likely slow) zero-out requests on backing devices that do not support discarding
+                 * natively, but do support zero-out. */
                 uint64_t discard_max_bytes;
 
                 r = fd_get_max_discard(fd, &discard_max_bytes);