Dual parity worth it with 8 discs?

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Right now i have 5 discs + a cache SSD. One of the 5 is a parity disc. I will be buying 3 more discs to fill my case soon. Thinking about using one of them for dual parity. I see many people with 8 or more discs running single so i thought i'd ask the forum's thoughts..  :D


EDIT: None of this stuff is mission critical stuff (Media). Only about 5% (the more "important" stuff) of it is backed up. I don't really care if i lose it all.. but i don't really want to either lol

Edited by superderpbro
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You can use the backblaze HDD reliability report to work out a rough % of failure rate for your model of HDD (or similar) and then compare that to your server. If let's say the failure rate is 10% then 1 parity out of 8 should be sufficient (12.5% protection). That should be sufficient for non-critical data.


For critical data, it's the number of copies and the locations of those copies that count, not number of parity disks.

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I put together a simple spread sheet a while back that gives a feeling for what the risks for an No-Parity, Single Parity, and Dual Parity setups.  Also include was some thoughts on other risks to your data.  You can find it here:





By the way, the BackBlaze Data is probably a worst case scenario for drive reliability because their servers' usage pattern is much higher than any unRAID server would ever see and the physical environment is probably harsher than that of most of our servers.  When their data release reveals that a drive model has a reliability problem, it is probably not longer available for sale.  The same can be said about drives with the lowest failure rates.  BUT it is the only source of data on drive reliability that is widely available and it does provide us with some feeling of what the failure rate of hard drives actually is.  It can provide with some insight into what the risks are for various configurations and with some knowledge of our level of risk acceptance that we are comfortable with and how to achieve that level. 

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👍 IMO good choice, it's not that uncommon users here having a dual disk failure, not a complete failure, but read errors on a second disk while rebuilding a previously failed one, even on small arrays, much more often than statistically data would let you believe, maybe in part because unRAID users tend to use and re-use older disks, and sometimes don't have notifications enable and/or ignore warnings.

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Note that it is still extremely important to setup notifications and (at least) a monthly parity check to check for easily correctable problems before they escalate.  Dual parity only provides for problems when recovering from a known single disk issue.  If you wait until you have a known two disk problem, you are still open to a potential third disk issue during a rebuilt!  The best guard against a potential data loss has always been a conscientious system administrator.  

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Good choice to go dual. 


In my case I had a cable issue and while I want to do a parity swap to a bigger one my parity drive had uncorrectable errors. I had notifications on to see warnings and was doing SMART from time to time. But there where no indication. One data disk was going offline while copying.


My luck it was a bad cable. I could have been a failed disk, then I had lost everything on it. With full backup no problem. But only the important files are backed up and the other ones had to be created again with a lot of work hours.

So this servers has now 3 4TB drives and 7 3TB disks with dual parity.

The other server has 3 12TB with one parity because I don't wanted to buy 4 12TB at the same time, quite expensive. So there is a risk of loosing some data and the important have  full backup. Even a dual parity is no replacement for a real backup. So I have to do backups on that anyway.


Great choice to go with dual parity imo.

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