vdev/pool math with combined raidzX vdevs...

[Jason Usher]

Hi Bob,

--- On Wed, 7/11/12, Bob Friesenhahn <bfriesen at simple.dallas.tx.us> wrote:

> The probabilty is indeed additive just as you say.  My
> point is that the fundamental integrity is offered at the
> vdev level.  If a vdev fails, then the whole pool is
> gone.  The MTTDL calculations for various vdev
> topologies vary by orders of magnitude, which tends to make
> the additive nature of more vdevs insignificant.


Thanks again for responding.

I'm not going to beat this to death, but just to summarize, if F is 2, then the corresponding data loss probabilities for RAID-Z1, -Z2, -Z3 are: 14.9%, 1.3%, and 0.086%.

But if combining multiple vdevs into a zpool (as opposed to maintaining a different zpool for each raidz3 vdev) is additive, then raidz3 becomes .258%.

Since (I think) a lot of raidz3 adoption is due to folks desiring "some overkill" as they attempt to overcome the "disks got really big but didn't get any faster (for rebuilds)"[1] ... but they are losing some of that by combining vdevs in a single pool.

Not losing so much that they're back down to the failure rate of a single raidz*2* vdev, but they're not at the overkill level they thought they were at either.

I think that's important, or at least worth noting...


[1] http://storagegaga.com/4tb-disks-the-end-of-raid/