The table file for the cluster mirror target looks like:

<start> <length> mirror \
cluster 2 <log device> <region size> \
<N> <dev1> <offset> <dev2> <offset> ... <devN> <offset>

an example might be:

#Example device mapper table file for cluster mirror
0 4724028 mirror cluster 2 /dev/sda1 1024 2 /dev/sda2 0 /dev/sdb2 0