vdev/pool math with combined raidzX vdevs...

[Jason Usher]

It's easy to find the failure math for raidz2 and raidz3.

But what if you create a pool with 3 different raidz3 vdevs inside of it ?

For instance, 3 12-drive raidz3 vdevs in one big pool.

For each individual vdev the failure probability is now higher, since not only will it fail when 4 drives in the vdev fail, but it will also fail if four drives in any of the other two vdevs fail.

So each raidz3 vdev now has a failure rate higher than vanilla raidz3 ... but what is that new failure rate ?  Is it still higher than vanilla raidz2 ?