vdev/pool math with combined raidzX vdevs...

[Jason Usher]

Hello Bob,

--- On Tue, 7/10/12, Bob Friesenhahn <bfriesen at simple.dallas.tx.us> wrote:

> > Am I really the only person worrying about the
> interactive failure properties of combining vdevs into a
> pool ?
> 
> Yes.  You are the only one.  The strength of the
> individual vdev is the primary determining factor of the
> strength of the pool.


Thanks for responding.  So I must be mistaken, and the failure probability of each vdev is not additive ?  As I mentioned earlier in the thread, I am not a probability person, nor would I trust my own calculations if I tried.

Because if it is additive, combining vdevs erases about half of the difference between raidz2 and raidz3, which I think is fairly significant.

Can we at least agree that it's not the same as a lone vdev ?  If I can destroy a vdev by blowing 4 disks in it, OR by blowing 4 disks in some other vdev, that's a higher risk than if it were alone...