+Note that this call checks for the existence of C<filename>. This
+stops you from specifying other types of drive which are supported
+by qemu such as C<nbd:> and C<http:> URLs. To specify those, use
+the general C<$h-E<gt>config> call instead.
+
+=item $h->add_drive_ro ($filename);
+
+This adds a drive in snapshot mode, making it effectively
+read-only.
+
+Note that writes to the device are allowed, and will be seen for
+the duration of the guestfs handle, but they are written
+to a temporary file which is discarded as soon as the guestfs
+handle is closed. We don't currently have any method to enable
+changes to be committed, although qemu can support this.
+
+This is equivalent to the qemu parameter
+C<-drive file=filename,snapshot=on>.
+
+Note that this call checks for the existence of C<filename>. This
+stops you from specifying other types of drive which are supported
+by qemu such as C<nbd:> and C<http:> URLs. To specify those, use
+the general C<$h-E<gt>config> call instead.
+