Parity Drive Question

[Guest]

Just a quick question about parity drives. When creating an array there are slots for parity drive selection.  So I can put in two 12tb drives, giving me 24tb of parity. So should I keep my array 24tb? Or am I missing an option somewhere? 

 

 

Sorry I had a hard time trying to explain my question when typing I hope it makes sense.

[itimpi]

The size of the parity drives is not related to the space in the array.   The rule is that the parity drive(s) must be the same size (or larger) than the largest data drive.   The number of parity drives relates to how many drives can fail before there is the potential for data loss.  

 

The question suggests you are not aware of how parity operates in Unraid so it would be a good idea to read the description of this in the online documentation.  An understanding of this will help make things clearer.

[Frank1940]

You can find information on how Single Parity works here:

 

     https://wiki.unraid.net/index.php/UnRAID_Manual_6#Parity-Protected_Array

 

There was a general discussion on Dual Parity a few years back and you can read about it here:

 

     https://forums.unraid.net/topic/51362-pq-or-dual-parity-in-detail/

 

 

Basically with Single Parity, you can have one disk in failure without data loss.  With Dual Parity, you can two disks in failure without data loss. 

 

I put together a statistical analysis of the risk factors for No Parity, Single Parity and Dual Parity a while back so that users could decide what the various risks were and their levels were.  (Disk failures are not that only way you can lose data!!!) You can read that thread here:

 

    https://forums.unraid.net/topic/50504-dual-or-single-parity-its-your-choice/

 

 

[Guest]

Thank you guys for your replay. I knew I wasn't putting the question together well. I think I understand it doesn't work like a RAID setup. The parity unraid uses is a bit out of my comprehension. 

 

So U guess a better way to put my question is: Is there an array size limit where the parity needs the second drive? Aside from having more drives in the array means more of a chance of having one fail, simply by having more. Feel free to tell me to just shut up and just have parity drives set up ( I do)

[JorgeB]

3 minutes ago, youcrew said:

Is there an array size limit where the parity needs the second drive?

No, like mentioned:

On 9/29/2018 at 8:19 AM, itimpi said:

The rule is that the parity drive(s) must be the same size (or larger) than the largest data drive.

 

[Frank1940]

3 hours ago, youcrew said:

Is there an array size limit where the parity needs the second drive?

 

If you read the thread that I referenced about about "Dual or Single Parity, it's your choice", you will find that the 'need' for parity protection is really about your level of 'Risk Tolerance'.   Only you can determine what that is.  Let's admit it, hardware will fail!  IF you are using Single Parity and you have two drives fail, you could potentially lose everything on those two drives.  With Dual Parity, you would not.  However, even with Dual Parity, if you have three drives fail, you could potentially lose the data on all three drives.  One way to decrease the possibly of data loss is to have a backup (Either the critical data or everything) that is on separate hardware that is off-site.  (Think about folks in the Carolina's, someone probably has a server that is under three feet of very muddy brackish water!) 

[trurl]

4 hours ago, youcrew said:

I think I understand it doesn't work like a RAID setup. The parity unraid uses is a bit out of my comprehension. 

Actually Unraid parity calculation isn't much different than parity in RAID or parity in general for other applications such as communications.

 

And if you take a look at that first link Frank1940 gave it isn't really very complicated or difficult to understand.