Mirrored Parity Drives?

[RussellinSacto]

Hey Guys,

 

Hopefully the super knowledgeable can educate me here...  Is it possible and wouldn't it make sense to mirror the parity drive for added redundancy?  This could act as a hot spare (without even requiring the rebuild).

 

Thanks,

 

Russell

[SSD]

I don't know the answer to your question, but do expect that it is possible to do this with a hardware based RAID controller.

 

I remember reading, though, that RAID arrays do not spin down under unRAID control.  You therefore have to weigh the advantages with the disadvantage of extra energy usage and wear/tear.

 

Also, I have yet to see anyone lose data because the parity drive failed.  It usually is related to some hardware problem (e.g., bad memory or a bad cable) that results in unRAID getting bad data and updating parity incorrectly.  If you have parity in a mirrored configuration, it would just update them both incorrectly.

[JonathanM]

The parity disk isn't any more important recovery wise than any other disk in the array. If any data disk fails, ALL data drives plus the parity drive are used in the recovery process. That's why regular health checks are a good thing, so you catch any drive starting to fail and take action before you have 2 drives go down. The concept of unRAID vs. say RAID5 precludes the usefulness of a hot spare that is only ready to take over for the parity drive. A truly useful hot spare would need to be able to take the place of any drive, and the act of trying to recover to the hot spare would tax the array by reading from every address on every disk, and if you hadn't kept tabs on disk health, you could very well be in a position to lose 2 drives. RAID5 suffers from this very problem, but it's fatal to the ENTIRE array, not just the failing drive(s). unRAID mitigates the risk to just the failed drives. Tom has talked about a different type of error correction that would be applied to a second "parity" drive to allow 2 simultaneous failures, but I don't know the status on that.

[RussellinSacto]

Jonathan....

 

Thanks...    I needed a headache. 

 

That answered my question perfectly...  Now I understand that simply mirroring the parity drive only helps if the parity is one of the two drives that fail....  If we had mirrored parity drives and lost two data drives, there's no way to rebuild that missing data.

 

I think I got it this time. 

 

Glad to know that Tom's onto the possibilities.  That would really be incredible.

 

Thanks again,

 

Russell

[WeeboTech]

The only drive that makes sense to mirror is the cache drive because for a certain period of time, data on the cache drive is not protected.

Other then that, having a spare drive available (in the machine not part of the array) or outside of the machine is the most cost effective strategy at the current moment.