[zfs-discuss] Re: raid10 vs raid-z2?

matthew.garman at gmail.com matthew.garman at gmail.com
Thu May 30 10:25:22 EDT 2013

On Wednesday, May 29, 2013 8:57:49 PM UTC-5, theki... at gmail.com wrote:

> Maybe worth checking the reliability calculator. 
> http://www.servethehome.com/raid-calculator/raid-reliability-calculator-simple-mttdl-model/
> I am probably guessing wrong but, will will have a backup right ? if you 
> do have a slow backup, then you should choose the fast configuration for 
> your live environnment. Even if it's less safe and completly crash, you 
> just have to restore from your backup. 

Backups, yes.  Basically, more disks in the same machine (but planning to 
move these to another machine on a fast local network; either way, 
reasonably fast backups).  But still---I'd rather not have to actually use 
the backups!

I've played with that calculator some.  I'd like to know more details about 
how it arrives at the numbers it does.  But for my particular configuration 
(six consumer-grade 3TB disks), it actually gives raid10 a bigger MTTDL 
than raid6.  In other words, it claims lower statistical probability of 
data loss with raid10 vs raid6.  Although if no data loss is the primary 
goal, raid-z3 is the way to go (which makes intuitive sense).

I know mdadm can do this, and I assume zfs can as well: "triple redundancy 
raid10", i.e. stripe across three-way mirrors.  But that's an expensive 
proposition, as usable storage space is N/3.  Not wallet-friendly.  But 
it's good for performance *and* data loss resiliency.
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