export srcdev=`rbd map rbd_ssd/vm-277-disk-0`; [ -z $srcdev ] && exit 1   # eg: /dev/rbd47
  export dstdev=`rbd map rbd_ssd/vm-472-disk-0`; [ -z $dstdev ] && exit 1   # eg: /dev/rbd48

  perl -'MDigest::MD5 md5' -ne 'BEGIN{$/=\4194304};print md5($_)' $dstdev |
    perl -'MDigest::MD5 md5' -ne 'BEGIN{$/=\4194304};$b=md5($_);
      read STDIN,$a,16;if ($a eq $b) {print "s"} else {print "c" . $_}' $srcdev |
     perl -ne 'BEGIN{$/=\1} if ($_ eq"s") {$s++} else {if ($s) {
      seek STDOUT,$s*4194304,1; $s=0}; read ARGV,$buf,4194304; print $buf}' 1<> $dstdev;