I swear I read somewhere it will protect data against two drive failure, now I cant find it. Although losing two drives at once fairly uncommon, its a security id like to have.

Yes, it only has one parity drive (think of it like RAID5) except that the data isnt striped, so lets say you lose two drives in an unlikely incident (parity drive and a data drive) - you'd only use the data on that lost data drive, the other drives would be fine.

 

There's been talk of adding a 2nd parity drive in the roadmap, but its way down the road. I hope it gets prioritized once V5 is final, because thats the next/last big piece that would lock me into never considering ZFS or another filesystem. 

Unfortunately yes.  We hope a second parity drive will be added eventually as when you start being able to have 20 drives, there is a chance of two drives failing especially on a rebuild.

Also.. and I maybe wrong here but at the moment, if there are parity errors, you can never be 100% sure if its the data drive that has errors or its the parity drive with an error.

Drives can randomly flip bits, bitrot etc.

With two parity drives, it would be able to compare both parity drives with the data drives and work out which drive is actually wrong which would be pretty awesome....

 

 

unRAID has single parity, which only allows for the rebuild of a single failed drive.

 

An enhancement, P+Q parity (aka RAID-DP/RAID6) would allow for a rebuilt to continue in the event of a second drive having a read error during the rebuild. This enhancement is not trivial. It would slow write performance and increase cpu load.

 

Two drive failures are rare, and multiple simultaneous drive failures are often not actual drive failures. They are often cabling, power, cooling, or connector type failures.

 

But as drives get larger, the probability of a read error during rebuild gets larger. Large drive capacity brings the requirement for P+Q.

 

Do an internet search on "end of RAID5" for several detailed discussions on the topic.

 

Choose your drive size wisely, it may save you data and power.

Where is unraid headed then in regards to this? Wouldn't adding a 2nd parity drive just make the write speed that much slower?

It should not be significantly slower. The second drive will likely contain a diagonal parity. Parity computation is trivial and the second drive can be read and written in parallel with the first.

Thanks for clarifying this. Its kind of a disappointment. Even though drive failures are rare, especially multiple, I am going to have a an arraignment with around 15 drives, and obviously with the more drives, the more opportunity for failure.

As internetfriend said the big advantage of unRAID though is that even with a multi drive failure you don't loose all of your data.  You only loose as much data as you loose in drives.  Other raid solutions will loose all data if you loose one or more drives greater than the parity protection provided.  So if you were to loose three drives out of 15 on unRAID you would still have 12 drives of data.  Raid 5 or Raid 6 setups would loose all 15 drives of data without going to a data recovery company.  This means that for large numbers of drives unRAID is actually better for data security then other raid solutions.  For me the reduced speed is worth the data recovery possibilities.  I don't have to send ALL of the drives out and spend tens of thousands of dollars+ to get my data back.  I only have to send the bad drives in and I don't have to pay for a RAID recovery just drive recoveries.  Since I only have recorded videos on unRAID currently I wouldn't even bother but I wouldn't have to start over again with my video collection either.  unRAID is a just the best way for data security even with a singe parity drive setup.  If we get 2 drive parity protection in unRAID it would become the best protection no question!

I have a 7 drive array (1 parity + 1 cache + 5 data).  I have had 2 drive failures in the past week, and had another a few months ago.  Fortunately, I lost no data because I was paying attention and had two spare drives laying around.

 

I would really like a second parity option.  Space is so cheap, another $80 for a 2TB drive for double protection is an easy cost to justify.

Space is so cheap, another $80 for a 2TB drive for double protection is an easy cost to justify.

 

A few weeks ago I would have said the same thing, but since the hard drive factories were flooded out in Asia recently that 2TB drive will now cost you over $200.  I really hope prices are back to normal within the next few months, but that will depend on how fast hard drive production get back into full swing.