check this, maybe could help to improve performace (it's not a raid1
solution, it's a cache solution, in kernel 3.9):

http://kernelnewbies.org/Linux_3.9#head-3dbc54b9324d21f06f55299b0a30d6cb06403529

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/plain/Documentation/device-mapper/cache.txt?id=c6b4fcbad044e6fffcc75bba160e720eb8d67d17