-r, --read-only
          Mount the filesystem read-only. A synonym is -o ro.

          Note that, depending on the filesystem type, state and kernel
          behavior, the system may still write to the device. For example,
          Ext3 or ext4 will replay its journal  if  the  filesystem  is
          dirty. To prevent this kind of write access, you may want to mount
          ext3 or ext4 filesystem with "ro,noload" mount options or set the
          block device to read-only mode, see command blockdev(8).